if you get a razorblade and VERY VERY carefully cut the zoas right where they meet the rock, they should be fine. The only problem here is that you chance cutting the anemone, which would probably kill it, and you also risk getting stung, so wear latex gloves and pull them up as high as they will go.
You may lose a few zoas, but if you cut them right at the rock, or try and get some of the rock with the zoas you should get most off safe