There are a lot of self-entitled people with big mouths that spout off because things dont go their way or others cramp their style. Boo-hoo. Welcome to todays society, not just here in the forums or in-game. Best thing to do, dont even bother with an FU. They are less than nothing, not even background noise, and not worthy enough to acknowledge their existence.
Now if they start badmouthing directly to others about you, then that becomes a problem for senior admins to deal with and quickly. This isnt a democracy nor is there free speech. As others have said about no personal attacks etc, all members have agreed to refrain from doing it the moment they joined, and if they break that agreement away they go and that is that.