This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
To guard against unwanted snails, use a weak potassium permanganate
solution. The Manual of Fish Health recommends a concentration of 10
mg/l as a 10-minute bath as a general disenfectant for aquarium
plants. Then rinse them in running water. This kills snail eggs and
parasites and might guard against algae spores.
Alum is also useful. Get "Alum U.S.P." at the drug store. Soak the
plants in a gallon of water that has up to 10 teaspoons of Alum. The
Alum kills microscopic bugs. Longer soaks (2-3 days) will kill snail
eggs and/or snails.