if they are in your tank already, you are going to have to remove them. I feel your pain. If the clams are easy to move around, I would remove them out of the water, just to keep them from falling onto your substrate. There seems to be some debate on the transfer and migration within the tank of these snails. I.E. whether they can cross the substrate, or glass, and whether or not they can trans-migrate between desirable herbivorous snails--eg. astrea or trochus. I've personally just been through this--I got a snail on a piece of rock someone gave me, and soon, every different kind of snail in my tank, and my clams, were being plagued by these snails. I've been removing the snails every few days and removing the pyramidellid snails with small tweezers. I HATE PYRAMIDELLID SNAILS!!!!!
I lost my red coris wrasse (juvenile, and I know you can't have one) a couple months back. He was wonderful at removing these snails.
In other words, get them out!!! In the Giant Clams book by KNOP, they are described in greater detail.