I finally had to ban mushrooms from the main tank for the same reason. They kept spreading, getting big and harrassing my other corals. They are all in the mantis tank now.
One thing I did for gsp when they were growing out of control was to use kalk paste to "prune" them. Just use a little syringe to put a thick paste on the polyps where I wanted to kill them. I had to be a little careful not to hit surrounding corals, but it worked great. You might try decapitating the mushrooms, then kalking them.