You can use a new razor blade and cut the foot as close as possible to the rock. I've also heard of slowly twisting and rocking the clam until it comes loose. This needs to be done very gently and can take upwards of a half hour. I tried it for about an hour and gave up....

 

Either what Hop said, or remove that piece of rock as well as the clam.

 

When I purchased my clam from someone it was anchored to a large rock, much bigger than the container I had to take the clam home in. So he took a hammer and chisel to the rock. He was able to chisel off a small piece of the rock that the clam stayed attached to.

 

Alot of flow or an ice cube.

 

I thought that only worked for nems???

I would chisel of a small piece of rock...if you tear the bissel threads it can cause the clam to slowly die.