Why are you killing them in the first place? I find most people bent on destroying them have received incorrect information on them. They've heard falsified statements such as "they'll eat all your plants" or "zomg they will take over your entire tank and you'll be completely overrun"
If you want to do it humanely and not as hands on as squishing, consider placing a container of water in the freezer until it forms a layer of ice. Break the ice and then drop the snails in the cold cold water.
If you want to do it humanely and not as hands on as squishing, consider placing a container of water in the freezer until it forms a layer of ice. Break the ice and then drop the snails in the cold cold water.