Stopping teamkillers can be done with a complaint system like suggested or by counting the amount of team damage etc. It will probably work, but it will also harm innocent people. I know it from other games, people have low hp, walk in front of you, then complain - probably out of being frustrated. Or with enemy territory they walk on your mine (no not planted in stupid place), and complain. Similar things will happen with tremulous, being a tyrant you teamkill someone blocking the way, but the person thinks otherwise and complains. Bottom line is that the best way to stop the real bad guys might be to start a vote or notify an admin if available.