One way, if you want to save and sell or trade the polyps is to take the rock out of the tank and rip/scrape/chisel them off the rock. Be prepared that the rock might break into pieces either intentionally or unintentionally. A screwdriver or cold chisel is a handy tool.
(Caution: Zoanthid and Palythoa polyp fluids can be noxious so wear glasses, wipe off the fluid that might squirt on the face, wipe hands often, rinse hands in the tank/sink and don't spend too much time at this in one sitting.)
Another option is to take the rock to MarksReef and exchange it for other rock and coral. I've seen tons of LR over the years. I know how a particular piece might seem irreplaceable, but believe me, there are always other beautiful pieces of LR that we come to appreciate as much or more than the one we are dealing with right now.
As for other options, for me, killing good coral is just not acceptable, especially when there are so many local hobbyists wanting it.
There is really no substitute for forward planning. If you don't want a coral to take over LR don't put it there to begin with. This is especially true for Green Star Polyps, Briarium, Xenia and Blue/purple Snowflake polyps, to name a few.
On the other hand, if you want several zoanthid/palythoa colors on a single rock, place them in a pattern where they butt up against each other and can only grow outward in a somewhat pre-planned direction.
Hope this helps.
Aloha
Edited by Mark Peterson - December 07 2014 at 9:23pm