They need to add an option for reporting for maliciously banning your teammate's champion that makes you lose your ban/ gives it to a teammate.
It's so annoying playing with someone who is bans your champion and says something like "dont play leona play janna". It doesn't help your team's chances of winning, since you're setting them off of their comfort pick and forcing them to play someone they didn't want to play.
And to people who say its no different if the enemy team bans it: intent is everything. It's the reason why feeding isn't bannable but intentional feeding is.