I usually put two rules in place before the game starts.

First: Each person at the table is responsible for enjoyment of all attendees.
Basically - don't be a jerk to people, and try to have everone enjoy the game. If someone doesn't want to enjoy the game, he can leave.

Second: If you start PvP without prior agreement with the player who is under attack (or you provoke it), congrats, your PC changed into NPC.
I have no problem with two PCs fistfighting or even duking it out if that's what they enjoy. But if one of them directly starts - he is overstepping rule 1 and as such - can leave.

I have yet to use more than "strict voice" and some fatherly advice to the players involved - never had to take away control of PC, but mostly because the rules are in place and respected.

Of course, this is invalidated in case of Toon or Paranoia...