1) throw away the sand. It's cheap anyway. (will have to rinse some new sand though.)
2) run the sand through a very fine sieve (may miss snail eggs however and other than a flour sieve, I can't think of anything fine enough)
3) boil the sand in a large cauldron (will work but is time consuming.)
4) add copious amounts of bleach and wait, then rinse thoroughly and add a triple dose of dechlorinator. (not good if you are like me and hate chemicals, and there is always a chance if the dose is too low, that it might not kill of all of that particular type of snails)
Keep in mind that the snail eggs are possibly in the plants, decor, filter, sump, etc. and any nets that went into the tank.