Just like the Community Staff can ban people, they are also the ones that can reverse a ban. People visiting any community must follow the rules (which can usually be found in the Community Description box -- click the Community name at the top of the screen), and they should defer all questions to the community staff. If you were banned from a community, and feel that the decision was unfair, please email email@example.com with details of what happened. In general, we will not override the community staff's decision to ban someone.
Remember, if you do not like the rules or staff of a community, you are always welcome to try visiting a different community, or you can even make your own community with your own set of rules.