Ropefish dispatch of snails very quickly, although I highly doubt you want them in your discus tank. They get big, and they are also generally very lazy.
Depending on your plants and if you have any wanted invertebrates, A low dose of copper salts (aquari-sol) can kill all of them and their eggs in 1 dose. Then it is just a matter of getting rid of the carcasses.
You can put a bunch of pomacea bridgsii,applesnails in.They will outcompete the pondsnails for foo,in a few of my tanks they have completely wiped out pond,ramshorn and mts.