Ignore them. You don't need to be exclusively on Facebook in order to deal with those who hurl insults and spread lies, I've witnessed plenty of that on this site, and I found that ignoring such folks is the easiest thing to do.

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..