Please do not put them in a pond, as they are potentially not native it can cause ecological problems + they can potentially spread to another watercourse etc.... I personally add Nerite snails to my tanks (and some amano shrimp) that both eat algae and do not leave much food for the other snails, this seems to naturally keep the population down. As long as you dont mind a small amount of them living in your tank. Otherwise, IMO, the most sustainable way to kill them is to crush them and feed them to your fish. Hope this helps!