You can try some ice cubes in a ziplock bag at the base of their feet, sometimes they detach. I've had the most success with a small ice pick and lots of patience being careful to slowly and gently lift and roll back small pieces of its foot. I've heard it's easy with finger nails, same method, but I dont have long enough nails to do it with. Seems like it would be less painstaking that way.
You could also just try to turn the rock upside down outside of the water, but above the tank and give it a few gentle shakes, sometimes they will release that way. I've had much less success with that route though.
Whatever you do, be careful not to tear the foot or they will not survive. If done correctly you can get 50+ for each single depending on size