Depending on the situation, if there's just a subset of the players involved, I've handled it a number of ways:
1) XP penalty for killing another player
Before I go on - - -most of these yahoos in my experience are doing this to assert their superiority - - 90% of the time when a player is telling me that attacking another PC is something their character would do, it's either wrong, or they've created a character that shouldn't exist. They need to know that their character is superior.
2) Have a wandering or set encounter occur directly after, or even DURING the PVP battle - this does a couple of things. One, it likely ends the fight...two, it absolutely establishes that PVP is a bad thing in your campaign, as it becomes quite likely that any PC deaths will be those PCs involved in the brouhaha. The only downside of this approach is the risk to other PCs, particularly if a PC didn't 'start it' but is still weakened.
3) Allow the fight, wait for one PC to kill the other, then turn that PC into an enemy NPC under your control.
"Yep, Bob, you were right, you WERE playing in character...but it's clear to me that THIS character you were playing was not a member of this PC team, but an NPC villain with other aims"