I "borrowed" this idea from @pjaj , it was in this thread: http://blocktopia.net/forum/threads/cant-kill-op.3141/
So credit to him, I just copied it into Zombie server.
I just added into it few words.just an idea. adding a message when you try to kill a Referee, it says "you cannot kill staff, while he is ref!" much like "you cannot kill teammates! in TNT Wars/" I think it would be a good thing, especially for new players trying to kill Referees.
So credit to him, I just copied it into Zombie server.