Really, there is no nice way to kill snails that are aleady born. I have heard of people boiling their baby/adult snails to kill them, but I would only do that with the actual eggs.
Do ramshorn snails lay eggs like apple snails? If so, be on the look out next time for eggs so you can kill them before they hatch.
I would say that squishing the babies would be the fastest way as long as you make it quick and get them the first time. It would be hard for you to do yes, but you need to think about them right now. The faster and least painful way is the best.
Maybe someone else has a better way of doing it. In the meantime, take a look at this site, they may have some info for you:
http://applesnail.net/