They can feed on other snails as well. Basically anything with a shell and inner "flesh".
One way to get rid of them is peppermint shrimp (some other shrimp work rather well too). Peppermint shrimp will eat aiptasia, which has nothing to do with this post, but are only known to eat the smaller ones because it's what they can fit in their mouth. If the problem is indeed an infestation of pyramid snails, which will never get past a couple of MMs, the peppermint shrimp will grab them, crush them and then eat them.
I have asked a LOT of people with clams and shrimp in the same tank if they have problems with pyramid snails and 95% of them say no. Do you have pepp. shrimp in your tank? I would be kind of surprised if the answer was yes. Unless the time frame was pretty far off from adding the shrimp and the snail problem.
Also, if you keep an eye on the side of a clams shell you might notice a "jelly" like sac that forms every now and then. The sacs are usually grayish in color. Pyramid snails breed at EXTREMELY fast paces. If you notice that sac, scrub it off before they have a chance to hatch.