I would buy a whole new tank!!!!!!!!!! I would love to have snails, just not with fish (But not pond snails... Well, In a different tank, maybe...). Hey, I'll take 'em off your hands! He he, no. What I would recommend would be to try buying a couple oranges, peel one, invert it and stick it in your aquarium before you go to bed, the same as the lettuce. Or, you could peel one orange, put it in your aquarium, eat the orange and wait. Haha. Or you could try selling them on eBay. I just bought 20 Malaysian Trumpet Snails off of eBay today!
Another theory is that you can throw some pennies in the tank. The copper is supposed to kill the snails. But first, find another tank or something large enough to fit all your creatures comfortably in, put the copper in, and wait till their dead. But make sure to wait and test your water before you put the others back in, and be sure to take the pennies out!! HTH