Turtles will eat them. So do weather loaches and Betas. You could try a Beta once the water temp gets reliably into the 70's. It will cost you a few dollars. If you have no fish, aquarium stores sell a copper solution called "Had A Snail" that will do it too.

Thanks kalevi,
Does the copper solution hurt anything else?

I used in an aquarium once to get rid of these conical shaped snails that burrowed in the gravel. It didn't seem to hurt anything but too much copper in the water is generally not good. You can google to see if it hurts anything else.

I meant a Betta would do for your snails if your pond is small.

Thanks kalevi.