Not as easy as that. We get a lot of good quality members that sign up using their facebook accounts. The game my site is about has an active facebook community run by the games developers, so a lot of people cross over using their facebook accounts.
You can actually do this with promotions. They traditionally are designed to add permissions, but you can use them to remove permissions. You do just need to accept the caveat that the promotion may apply after the criteria doesn't match (until it's checked via cron) or not be applied when it starts to match (if it matches at registration, it should be ok).
You'd select the criteria "User's forum account is associated with their Facebook account" and "User has posted no more than X messages". While they match, add the user to a group that you've set some "Never" permissions for (to forcibly remove them).