Removing Star Polyps

Ok so awhile back I got a frag of star polyps. After reading up about them I learned that they are very easy to grow and can over take a tank. So I placed the frag on a rock and kept the rock away from the main rock structure of the tank. The rock is now completely covered in the polys and they have encrusted on to the bottom of the tank (I run bare bottom). I would like to prevent them from spreading further but not destroy the main colony on the rock.



Any suggestions on how to handle this? I was thinking of using a razor blade to separate the main colony from the bottom of the tank then scraping the remaining polyps off and sucking them up with a siphon. I would just like to keep the one rock with the colony and not have them spread throughout the entire tank...