Banning your teammate's champion can often win you the game if you are doing it with strategic motives rather than immature motives.
It has worked for me a lot. If your teammate is about to pick ivern top, ban it. There isn't really a pick that can get much worse than that. If they intend to throw the game afterward, they would have thrown it anyway with such a stupid pick.
And to answer your question and give you my own thoughts.... People who intentionally throw games are the only people who should be banned. Chat logs shouldn't matter with the existence of a mute button. Also, Riot will never tell you if the other person is punished or not even if they actually committed an offense.