To get zoas off the base rock, pry them up with a knife. It may be difficult to start. Once you have a bit loosened from the rock, it should get easier, but it's still tedious. When you have what you want for a frag, just cut it from the rest of the colony. If you're concerned about releasing too much toxin, just don't make more than one frag at a time.
BTW, even if you scrape off the entire colony, it will probably grow back. For this reason, some people culture their zoas on the sand or on islands, and not on their base rock.