Clarabell,
First, you might be interested in
this thread which discusses many of the issues of non-combat activities and how to meet their requirements.
I'm more of a noncombat style myself, and have also dealt with the ultraviolent player before. In this case, killing the innkeeper over and insult, and stabbing the homeless, I've got a number of pointers that might help.
Never forget about consequences.
We tend to choose the nonviolent action in real life all the time. Why? Consequences. First, there's the physical damage, without curative magic. Second, there's the law and its overwhelming power. Third, there's the social stigma. If attacking the homeless in game will get you hunted by the guard, no friends, and guaranteed defeated, you won't attack the homeless.
Be aware of status games.
One of the reasons that violence occurs even when you try to avoid it is becuase the perpretrtators feel weak and low status. By finding someone weaker and beating them up, they feel better about themselves. This is usually what happens behind an attack-the-beggars scenario (note that it's not a mug-the-shopkeeper scenario, where the rewards are potentially better). Similarly, the insult threatens to lower the PCs' status, and so they respond by asserting superiority. Violence is expedient. Since part of a fantasy game is to enjoy status you don't have in real life, some of this is to be expected (but not necessarily appreciated). So, to calm a bloodthirsty streak, give them chances to feel a little more important anyway. Find a few things they can be successful in, and let them be successful. Let them have a little power. Heck, even a cowering flunky can do much to calm someone down (it's why the cowering's done in the first place).
Be careful of your own power.
You may not want things to get violent, but make sure that in the process you do not
forbid violence. If the players think you're making a particular activity impossible, then they'll try to do more of that very activity. It's like the cursed PC from earlier--you made all his evil turn into good, so he felt the need to try to commit greater and greater evil, far more than he would have tried without the curse. If the players start to feel that you're trying to make sure they don't have combat, they'll rush head first into combat and do everything they can to make sure it happens.
Understand what the players want.
Seriously, they might just want to fight stuff. Don't say no to that. Just make sure that they direct the violence in productive ways through storycraft. If you can't stand their play style, find new players. It's as simple as that.
Make sure noncombat solutions are worthwile.
If they're used to fighting to be successful, you will have to spell this out for them, in the beginning. Basically, though, you'll want to make sure that there is a task that needs completion, an obstacle that needs overcoming, and a reward for completing the task. If you only have two of the three, it beocmes far less interesting. Example: Your village is dying from the plague. You have found out that there is a cure for the plague, but that it is held by a dragon. If you can get the cure by just asking the dragon politely, it's no fun. If you have to fight the dragon, it's fun. But if you can also make a wager of riddles with the dragon, and he'll give you the cure if you win, then it's still fun. If you have to find a way to trick the dragon, it's fun. If you need to perform a grueling service for the dragon for the cure, it's fun. If you have to sneak into the treasure room while the dragon sleeps, it's fun. Being able to say you saved the town because the dragon's really a nice friendly guy who let you have it for free garners no status. But defeating the dragon in some way and saving the village garners a lot of status, and feels good! Whether or not there's violence, it's still empowering.
Just step back, ask why the violence is occurring, and make sure there are interesting challenges without combat and you'll do fine.
