Not entirely sure as never tried but can in the week before remove rocks remove them one by one as they are grabable and any left after the week some kind of flush over a bucket or tank with pouring saltwater over the rock which is out of the water but snails stuck in rocks will be a tough one im sure.
Or if could hold the rocks up with some kind of string slightly out the water and hopefully they move towards the water each time.ovbiously dont stand there holding the string,you would need something above the tank or bucket of water to hold string and slowly raise rock every so often.
Not sure if this would work just an idea that quickly popped in my head.
Other than this its going ne break the rocks up but im sure someone had this problem before and a solution so good luck