I can ignore the dimwits but the really smart ones are hard on the ego

Block and move on. Not worth wasting mental energy unless it happens to be a real person you know that you can't do that to because of work or whatever. I find real acquaintances rarely do that, even if we don't see eye to eye.

Report to Facebook if it is out of line or admin if it's a group. Be careful reporting to Facebook if it is a group because it can get the group shut down rather than the troll. It's a common tactic to infiltrate groups you don't like and to bad things in order to get the group shutdown. Theists do it to atheist groups and probably vice versa..