Earlier quoted context omitted.
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.
Like r/the_donald. Mods can effectively whitelist a set of users by banning everyone else.