Remember that if you delete a comment that a lot of people have responded to, it'll also delete all of those replies. If you don't want to remove the replies from Facebook, you might want to consider editing your comment rather than deleting it.

What happens when you delete a Facebook comment?

Deleting the Facebook comment will erase it; no one will be able to see it. The user will know that the negative comment has been deleted if they go to view it again, but they won't be notified of its deletion. Banning users will keep that particular user from commenting on any of your posts ever again.

Does deleting a comment delete the thread?

Yes, deleting the first comment of a Facebook thread will also delete the thread.