I've seen a plug-in for server, which giving killed teammate to bust killer from same team. You can take 50% of his money. Forgive to him, and say that was a mistake or give him warn (2-3 warns =ban) It would be better, because sometimes allies got killed by bad luck , or something. i think that something like that should be at RANKED matches, coz this actual ban system is really...really bad.
Sorry for bad English, still learning, and i won't to use google translate :D