A build mode always griefed

[Xaxime]

Everytime when I launch or I go to a build custom game, I see a player named "floor" (which has other names) who destroys all the buildings. He uses all the ways to grief all the map and to not be banned : rapid grenades, /df command, votekick cheating, he goes away when he feels a ban and returns later fucking all the players - sometimes with a different name... So then honnest players get fed up and quit the server, which is shut down. This mode is becoming unplayable !

Have you ever cared or heard of this problem ? What can we do to stop this griefer ?