I'm not sure how many of these pests anemones your dealing with, but if you can remove the rocks with ease you could always burn them off with a cigarette lighter. It only takes a matter of seconds sometimes and that black residue left behind usually rubs right off. Definitely something to think about. GL.
Some copper banded butterfly fish will eat aiptasia, some file fish will, some peppermint shrimp will. None of these are reliable in this regard. Berghia nudibranchs will. Racoon butterfly fish will but are not coral safe.