The admins. The people who run the server the community is hosted on.
Ideally, yes. But each instance gets to decide who they ban. If there's an AngryNazi.fuckyou instance, each other instance can decide they don't want to talk to them or see their communities. Don't want your instance to get banned on another instance? Control your users.
I guess? If your a user of AngryNazi.fuckyou and that instance gets banned in a lot of instances, you will need to make a new account in a less tainted instance.
Unlimited (depending on hardware power Vs users, of course)