I have a yellow bellied blue damsel in my tank. He does not bother anything - not even the ocellaris clownfish (of course, they are the Spawn of Satan themselves!!!) My tangs and angelfish push him around during feeding time. He feeds on left overs. I would move him out if you are planning on keeping smaller, less aggressive fish (firefish, gobies. pipefish, anthias, etc). Anything smaller will be toast. Especially if you have a really pugnacious bastard, like a yellow devil so something like that.
If you are dead-set to get rid of it, this method works everytime (for me anyway). It takes a bit of patience, but stick with it. take your net and put in in the water, facing your powerhead so it keeps flow of water into your net. put in a cube of frozen food (or a half a cube if you don't to add so much nutrients into your water) and it will keep pushed into the net, melting and smelling yummy to your fish. They will sneak up to the net and try to eat off the back (STAY STILL) and eventually, they will wander into the net, since the current from the powerhead is pushing all the food into the back of the net. The damsel will also get a slight push into the net when it hits the current and POW yank the little fu&%#r out of your tank and flush his nappy-ass down the toliet. It if doesn't work the first day, wait til the next day. I have never had to try it more than twice. Best of luck to you.
I have one more idea, but I will save that one on the off chance this doesn't work.
-Dr Marco :sfish: