My experience in this area is that GSPs can be nearly impossible to stop, once they "take off." I know you said the rock is deep within your reef, but isolating the colony onto a rock "island" in the sand is about the only way of being sure it doesn't take over your tank. The rock island can be any size/number of rocks you want, as long as it is surrounded by sand. The GSPs will eventually begin plating out ono the sand, and are very easy to harvest at this point, preventing their spread to surrounding rocks.
If the above is not really possible, I've found that MH lights really seem to slow the growth of GSPs (compared to PC and VHO lighting). The only coral I've found that fought GSPs to a standstill was hydnophora, but their might be others with a powerful sting that you can "corral" the GSP colony with to help prevent the spread (like maybe galaxea? - haven't tried that one, so don't know if it would do the job or not).
I know you may feel that the GSP rock (or rocks) in the midst of your reef really isn't much trouble, but after having a 2" GSP frag take over 2/3 of a 40 gallon tank, overrunning various corals in the process, I firmly believe that advanced planning can save a lot of headaches later on.
FWIW,
Kevin