The "rock" in your tank isn't stone. It's a porous structure mostly made of calcium carbonate and other solids. If you grab a piece, you can usually break it by hand. So in the case of zooanthids, you can take a clean knife, grab the rock, and cut into the rock just under the zoa base. It's kinda hard to explain on here, but it's exactly like whittling a piece of wood with a pocket knife. Just be sure to wear gloves and googles. The other thing you can do is get some bone cutters or side cutters and cut out the piece of rock the zoas are on.
