Load up one of those old skool punishment systems where someone who got tk'ed can do a number of shenanigans to their killer. Perhaps even making this scale so if someone kills several people, the punishments could include lengthier bans. I also seem to remember from the 1.3, 1.6 days, a way to make it so somebody dies if they teamattack, not that something this extreme is good, but perhaps after a number of team attacks, lets say 10 or so, this becomes active for that player. If you ruin their fun, they won't stick around.