Bear with me, this might get long.
Sometimes, you just have to tell the players: "No killing each other or stealing from each other in this game (anymore)." That alone is usually enough to get players to stop fighting, because if they fight each other then you just stop the session and don't adjudicate the fight. If they just want to have practice fights against different characters, tell them to do it on their own time, not during the session. Now, most groups don't need to be told this, but apparently yours does.
That's the simplest solution, but not always one people like. Sometimes sacrifices like "cooperation" have to be made though to promote the better enjoyment of the limited session you guys have to play.
Now for the more elaborate and subtle solutions. I will just present a variety of possible answers for you, you can use one, some, all, none, whatever:
Ask them why they are killing each other over loot. Emphasize that it is a cooperative game, and that the more they work together, the more treasure they will be finding for everyone -- so it is really a waste of time to fight over the bits they have already found.
Why do you think they are like this? Are they all just goofing around? Sometimes that's OK for a game, but often at least some members would like to play the "normal" way.
It's possible they are starved for "action" and when not presented with good targets to kill, they turn on themselves. Make sure they have a clear way forward and that there is promise for fighting and looting. Also make sure they know that it will be tough to overcome those challenges.
Make sure that all party members are useful. Vary your encounters so that each character gets a chance to shine. Lots of minions every now and then for the Wizard, Brutes with poor AC for the Strikers, Elites and Solos for the Defenders. Also put some traps in every now and again for the high-perception thievery characters. You don't want anyone to feel unnecessary to the group's successes.
Consider having them make shared backstories, such that they were all friends from a village, or are all working for a good-ish organization that won't look kindly on pettiness or murder. If their characters are friends as opposed to perfect strangers just adventuring for loot, they will be less likely to kill each other and more likely to share.
Give them quests to go on with goals, not just "kill things and take their stuff" dungeon crawls. It can greatly change the mindset of the players by making it a cooperative endeavor as opposed to a free-for-all. Make sure there is a good reward to the quest for each of them individually for when they return successful (don't make a big pile of money, make sure it's a certain salary each).
Tell them you would prefer they cooperate because it is reducing your enjoyment of the game, and that you think they would have a better time working together rather than fighting.
Forcibly institute a loot system for the group if they are unable to come up with a fair one. You might say all cash (including art objects) is split evenly (overage gets put aside in a shared account until it can be shared evenly), and that items should go to the player who needs it most. If they are in a party mindset, they will be able to determine who needs what the most.
Make sure that the found loot is fairly specific to individual characters. If the Ranger is the only one who uses bows, then the magical bow would obviously go to him. If the Fighter is the only one using scale armor, then found magic scale obviously goes to him. That should reduce stealing because there is little to gain by taking items you don't/can't use.
Try not letting them sell magic items. If nobody will buy, then there is no incentive to hoard items to themselves. Let them disenchant the items for residuum and make some new things that are useful, but don't indulge in allowing them to steal for profit.
I'm sure there are lots of other solutions too.
Now, as an aside, it is really very hard to kill your own teammate unless you just withhold treatment to a dying character. PCs don't die until they reach their bloodied value expressed as a negative number; if they had 50HP normally, they aren't dead until -25 or until they fail 3 death saving throws while "dying."
Sometimes, you just have to tell the players: "No killing each other or stealing from each other in this game (anymore)." That alone is usually enough to get players to stop fighting, because if they fight each other then you just stop the session and don't adjudicate the fight. If they just want to have practice fights against different characters, tell them to do it on their own time, not during the session. Now, most groups don't need to be told this, but apparently yours does.
That's the simplest solution, but not always one people like. Sometimes sacrifices like "cooperation" have to be made though to promote the better enjoyment of the limited session you guys have to play.
Now for the more elaborate and subtle solutions. I will just present a variety of possible answers for you, you can use one, some, all, none, whatever:
Ask them why they are killing each other over loot. Emphasize that it is a cooperative game, and that the more they work together, the more treasure they will be finding for everyone -- so it is really a waste of time to fight over the bits they have already found.
Why do you think they are like this? Are they all just goofing around? Sometimes that's OK for a game, but often at least some members would like to play the "normal" way.
It's possible they are starved for "action" and when not presented with good targets to kill, they turn on themselves. Make sure they have a clear way forward and that there is promise for fighting and looting. Also make sure they know that it will be tough to overcome those challenges.
Make sure that all party members are useful. Vary your encounters so that each character gets a chance to shine. Lots of minions every now and then for the Wizard, Brutes with poor AC for the Strikers, Elites and Solos for the Defenders. Also put some traps in every now and again for the high-perception thievery characters. You don't want anyone to feel unnecessary to the group's successes.
Consider having them make shared backstories, such that they were all friends from a village, or are all working for a good-ish organization that won't look kindly on pettiness or murder. If their characters are friends as opposed to perfect strangers just adventuring for loot, they will be less likely to kill each other and more likely to share.
Give them quests to go on with goals, not just "kill things and take their stuff" dungeon crawls. It can greatly change the mindset of the players by making it a cooperative endeavor as opposed to a free-for-all. Make sure there is a good reward to the quest for each of them individually for when they return successful (don't make a big pile of money, make sure it's a certain salary each).
Tell them you would prefer they cooperate because it is reducing your enjoyment of the game, and that you think they would have a better time working together rather than fighting.
Forcibly institute a loot system for the group if they are unable to come up with a fair one. You might say all cash (including art objects) is split evenly (overage gets put aside in a shared account until it can be shared evenly), and that items should go to the player who needs it most. If they are in a party mindset, they will be able to determine who needs what the most.
Make sure that the found loot is fairly specific to individual characters. If the Ranger is the only one who uses bows, then the magical bow would obviously go to him. If the Fighter is the only one using scale armor, then found magic scale obviously goes to him. That should reduce stealing because there is little to gain by taking items you don't/can't use.
Try not letting them sell magic items. If nobody will buy, then there is no incentive to hoard items to themselves. Let them disenchant the items for residuum and make some new things that are useful, but don't indulge in allowing them to steal for profit.
I'm sure there are lots of other solutions too.
Now, as an aside, it is really very hard to kill your own teammate unless you just withhold treatment to a dying character. PCs don't die until they reach their bloodied value expressed as a negative number; if they had 50HP normally, they aren't dead until -25 or until they fail 3 death saving throws while "dying."
