You can try chemical treatments (copper-based) if you're really desperate, although I would only recommend those as a last resort. Either make a snail trap and remove them manually, or get assassin snails to kill them off. But tbh if you keep the tank clean and don't overfeed, the snails won't multiply out of control. Most established tanks have snails in them & usually they don't pose much of a problem.
I wouldn't add alum directly to the fish tank, it is best used as a dip, and you should rinse the plants carefully to remove any remaining powder before you put them in the aquarium. You can also dip new plants in potassium permanganate or hydrogen peroxide to kill off any external parasites/snails that might be attached to them, but some of the more delicate plants might not always react well to this, so be careful.