Do you have any fish in the tank? If so, I'm surprised they haven't been eating the seed shrimp. As far as the snails go, their population is controlled by how much you feed the tank. If you overfeed, you will likely have a population explosion.
There are chemicals out there to eliminate snails but they will cause a big ammonia spike due to the decaying dead snails left in the tank. Chemicals to remove snails will also eliminate shrimp. I assume you have shrimp since you are posting in the Shrimp/Invertebrate forum
Try putting a blanched piece of lettuce or spinach in the tank. Go back a couple of hours later and it will have plenty of snails on it. Simply remove it from the tank, snails and all. If there are shrimp on it, they usually jump off before you remove it from the water. Do this a couple of times and you should be able to eliminate or drastically reduce your snail population. I do this about once a week in my shrimp tanks and feed the snails to my Tanganyikan tank. The tretocephalus eat any snail that will fit in their mouths, shell and all!