Short of throwing out your rock, it may be possible to encourage them to release from their rocks by making the local environment unfavorable for them. Here's a couple of ideas - Try directing very high flow against them so that they are really getting buffeted. They may release with time. Or adjusting their lighting. Maybe "light's out" in the aquarium for an extended period of time? If not, how about rotating your live rock so that they are turned in towards darkness, or maybe put the most affected rocks in a bucket in the dark with nothing but a pump to circulate the water. Or how about injecting kalk into the foot instead of smearing it all of over them. If you can get the foot to release you can you can pluck them off. When they're not happy, I've seen discosomas in my tank release from the rock that they are on and roll around in search for another place to settle down.
-B