I wouldn't recommend new plants until you've got them under control--they'd just eat them. Lower the temperature as much as is healthy for your fish and starve the snails out. Keep the tank dark to discourage algae, and vacuum the gravel often. Remember not to overfeed your fish especially. Snails won't reproduce if their living conditions don't warrant it.

They have pesticides for fish tanks, but that would kill any other invertibrates you're keeping as pets, and I don't know how safe they are for fish in actuality. Might be harmful to your bacteria, too.