You know, there's a system in Left 4 Dead that I really like, but I've never seen in any other game. At any point during a game in L4D, any player can vote to kick a player from an activity. If enough players vote yes, that player is kicked from the activity after a second or two. In PvP, only the player's team is allowed to vote. While this could create new problems altogether, it could help a lot if done right.