Quite strange such thread has not been created yet, time to make up leeway.

For now the only defensive mechanism against teamkillers we have is voting to kick a player which can be initiated by anyone. But often people dont want to waste game time for voting - on those occasions when someone started voting for kicking somebody I didnt even see where and how to vote.

Perhaps some "quick" punishments could be introduced? Say, when you're killed, instead of forgiving your comrade you can press another button and punish him - say, he gains highly diminished stats for his characters for a time being or simply dies in return and cant respawn for a time being, something like that. Some hidden count of teamkills can also be implemented so if you occasionally killed 1 or 2 comrades in a turmoil of battle you wont be punished but when you do so too often then vengeance is unavoidable.

Several punishments which make a game very unpleasent for a violator might be enough to correct their behavior and improve fighting skills.

There is a second system already in place beside /vote kick xyz. It's the forgive system. When you are teamkilled you can press I think it's E if you want to forgive that player for the teamkill (there will be a white text on the top of the downed state screen). If repeatedly the player will not be forgiven for his teamkills he will be automatically kicked. The amount and frequency of teamkills is taken into account in the algorithm.