i have heard reports of people wiggling them VERY VERy gently for up to half an hour for it to release itself. if you cut the bysass threads (i believe that is what they are called.... havent researched clams too much since right before i got one. haha) AS FAR AWAY FORM THE CLAM AS POSSIBLE it is possible for the clam to survive. but if you arent gentle enough or if you cut too close, the clam will die.
your best bet is to break the piece of rock they are on IMO, but the two topics above have been done. i would like to note though, that i have not done either of thsees, jsut read about them and that it IS possible to kill your clam doing it.
next time make sure to put the clam on a smaller piece of flat rock or clam shell so moveing can be a breeze