Right, so it's a bit more coarse then banning or allowing users, but if you selectively ban or allow topics that certain kinds of users want to talk about (or don't want talked about), you can definitely shape your user base.
Selective banning carried out by reacting to infractions is is a relatively coarse tool compared to whitelisting a set of users on the basis of credentials or avowed common interest.
Of course they can.
They simply ban/allow people until they get the mix of people they want. It happens all the time on Reddit e.g. /r/the_donald, /r/conspiracy.