Agree with Mr.Salty - Toss them in a pan of boiling water for a few minutes.
I have several "nice" small seashells in my tank, and figure if you keep it to a minimum, would be okay. Most of mine are just flat shells, would think that the coiled up ones like snail or those huge conk (sp) shells may harbor some nasty stuff inside, but boiling should take care of it. Seashells are just part of natural system, and will slowly breakdown over time and add a little calcium to the water.
Broomer