I have been completely unsuccesseful in removing mini-ramshorn without poisoning via copper.
They are the plague. I have had them survive bleach and potassium permanganate dips. I finally got rid of them in a 10g quarantine tank (which I had bleached... wtf), via copper. I haven't tried the H202 method, suggested above, yet. Sounds promising to me though. Either method would requiring removing the snails you wish to keep.