The snail shells of freshwater snails aren't very useful at all. I would assume you have a snail outbreak in your tank? We thought you meant the shells of slatwater snails, clams, etc. These are sometimes added to raise the ph slightly. Freshwater snails usually have a much thinner shell.
Just take the snail out of the tank. The body of the snail does decay and can produce some negative effects. Its similar to leaving a dead fish in the tank. But its not quite at the same level because of the size difference.
You might also try and destroy the eggs before they hatch. I'm sure you can find some if you look close. They are clear and almost transparent.