I would use potassium permanganate as a plant dip (10 mg/l for 10 min). I would never add it to the main tank. The high concentration of manganese released by this treatment may cause future problems for plants and invertebrates.
That said, I think you are better off working with your snails rather than trying to kill them. I consider snails to be helpful in an NPT. They are great for cleaning plant leaves and recyclying fish wastes. If you feel you must get rid of them, you can bait them or get fish that will eat them.