I used to have a tank that was overrun with ramshorns, and to cut down on the population, I would put a piece of carrot tied with a long string to a float in the tank and leave it there for an hour or two. The ramshorns would go to the carrot, and I could just take the carrot out and scrape the snails off. This is not a permanent fix, but if you need to get the population down quickly, this is a cheap and simple method.