I don't know about RBTAs specifically but a lot of people say holding ice to the foot (ro/di ice) in combination with massaging the foot of the anemone off the rock will help get it off... but go very slowly so you don't rip the foot... I've had success taking them off of glass but haven't tried rocks..
Use a piece of metal like a flathead screwdriver or so.
Nems HATE metal.......... just hold it up to the edge of their foot and slowly it will let go.
Move the screwdriver all the way along the edge of the foot and in a few minutes it's off the rock.