I just make sure that their rock is tight between other rocks, and if they want o move they will.
Ive tried prying them off, I have one that wants to move, but cant reach. He must be going nuts because he hits it with his head, but cant reach it with his foot! I couldnt safely remove him from the rock he was on, even though more than half his foot was allready up. Maybe the trick is if they hit their head occasionally, they will try and find it with their foot.
hth!