Well, there are a few ways that seem to work pretty well. If the rock it's on is too big to get out of the water, direct a powerhead at the anemone's foot. Nothing too harsh, but enough to make the anemone want to move out of the way. Icecubes sometimes work too. If the rock can be lifted out of the tank, pick up the rock and turn it so that the anemone is upside down and dip the anemone/rock into the water over and over again (probably 5-10 minutes). It should let go then