You have a couple options you can try. They both involve "poisoning" the snails but making your tank easy to make safe for re-use in the future. You will end up killing the bacteria in the tank as well, so you will need to re-cycle. But I think is a better option than bleach.
1. Get some Kent Poly*Ox and use a very strong dose, several to many times what is called for. Mix the sand up well once the water is a nice deep purple. After a day the water will be yellow, which means the potassium permanganate has been oxidized. If the dose was strong enough it will have killed all the snails. do a couple water changes, and your tank will be good to go.
2. Dose the tank with extreme amounts of ammonia (the same stuff used for fish-less cycling - Ace 10% janatorial strength). If you add enough to your tank to smell it (that is smells like a janitor's mop bucket, you will kill everything in the tank. Give the sand a good stir and mix. Do lots of water changes and test the ammonia levels <4ppm and then you can start cycling right there.
Option 2 may be a bit more noxious, be sure to ventilate the room until you remove the ammonia water, but Option 2 costs less, maybe be easier to find than Poly*Ox, and starts your cycle right away. Option 1 is safe, oxidizes all suspended organic matter and will kill invertebrates (and vertebrates) at a high enough concentration. If you happen to spill some Poly*Ox on the rug or furniture or your pants just rinse with hydrogen peroxide and the stain will dissappear.