Kha'Zix: Banned because if he gets snowballing, he can destroy teams with massive AoE damage. In addition, he's put mid and often easily beats most mages.
Eve: I'm guessing because it's a residual from back when she was still really powerful (I still didn't find her ban worthy).
Darius: He jumps on mistakes really easily and not everyone knows how to play against him. If you don't play top, people ban him because they don't want to deal with the fact that his teammates can't play against him.
Some other common bans are: Malphite, Morgana,Katarina, and Cho.
Kass doesn't see many bans as he's easily countered and Ali isn't really banned any ore as his jungle presence has been drastically reduced. In my opinion, Blitz, Mumu, and Kat all have the Darius effect.