Lock Banning of Pre-Selected Champions.

Simply put, if another player on your team has the intention of playing a certain champion and has pre-selected them, none of your other teammates can ban that champion.

Sure, it opens up the possibility of trolls pre-selecting a troll champ, but if they're trolls, they're not exactly going to care if you ban their jungle Soraka.

I think it's best to prevent the rampant griefing by other players though. Soloqueue's ban system should do what it can to eliminate trolls ability to mess up games.

Just because it won't solve ALL troll problems, doesn't mean it won't help. :)