You can't permanently block anyone from any website. They simply need a new username and email address to re-register under a different name. (A few websites might also monitor IP addresses but you can change your IP address simply by switching off your router and then turning it back on again, or by routing your internet traffic over the Tor network).
The use of moderators only works when you can be sure that most posts will be seen by at least one of them. AB, for example, probably has no more than a few hundred regular users, plus some casual visitors. Each of them probably only posts a few times per day, at the very most. So roughly 15 moderators can keep an eye on things reasonably well.
Facebook, however, has about 1½ billion users, many of whom might post dozens (or hundreds) of times per day. So where would all of the moderators come from?