it may seem odd, but try adding an apple snail. I recently acquired a hermadiphoric snail in a bunch of moneywort I purchased, and it busily went around laying egg cases. well, my resident columbian ramshorn snail quickly and completely devoured each egg case he found. I don't know if any survived, but I presume he may attemt to eat any baby snails, too. I have moved the snail to my 5G, as they are beneficial to aquariums IME and I want them there to help control algae.
I do not reccomend had-a-snail. it does contain copper sulfate and will kill inverts IME. also try emptying out a 20 ounce soda bottle and sticking a lettuce leaf inside it. it works better than the leaf alone because only snails can enter and no one else can, and the snails have trouble leaving once they enter.