In the new Draft Pick champion select when you're allies ban you're preferred champion.
That should result in a warning since its just rude,selfish,negative.
-Example: on PBE one guy wanted to play Jhin but forgot to set him as a preferred champion and I did,so he asked me if he could play because he didn't play him at all and I said me neither and he got the banning turn and banned Jhin selfishly.
I think that should result in 1 or 2 warnings and after that a punishment to not be able to ban for a few games and if that continues not being able to ban for a longer time.(forever if needed) or at least make it so just 1 guy bans again so there is less chance of that happening again.