In Facebook groups, you can ignore someone, but the person in question can still participate in discussions that don't involve you, or discuss what you've said outside of your own threads.

I believe Eliezer was referring to starting posts. So the question is, which is better, a banhappy omnipotent OP or gradual undoable community moderation?

counteracting mass downvoters (who apparently have gone to the extent of downvoting someone over 1000 times) is a huge burden on other people and not something they should have to do.

And indeed, it's not something that happens often. Eugine is so far the only person to be banned for mass downvoting in the ~5 year history of a very active site.