use a trap. This one is easy as pie to make. I had a coral nipping damsel that ate almost all of my poor goniopora, and when I trapped it, I was relieved. It may have to sit in there for a few days, and you may catch the wrong fish, but there is a trap door to let them out. I wish you luck!
http://www.instructables.com/id/EDLCFKOYY8EPLK0ED1/