The short answer
You get rid of pest snails by removing what feeds them, not by attacking the snails directly. Cut back feeding, remove snails by hand, and if you want a biological solution, add assassin snails or a snail-eating loach. Pest snails (bladder, pond, ramshorn, trumpet) boom because of overfeeding, so fixing the food supply is what makes any other method stick.
Step 1: Cut the food supply
Every extra flake feeds another snail. Feed only what your fish finish in a minute or two, and siphon out anything left over. Within a couple of weeks a starved population shrinks noticeably on its own. This step matters more than any of the others — see how often should I feed my fish.
Step 2: Remove them manually
Crush snails against the glass (fish eat the remains) or pick them out during maintenance. A classic trick: drop a blanched courgette slice or a lettuce leaf in at night, and lift it out in the morning covered in snails. Repeat for a few nights to make a real dent.
Step 3: Add a natural predator
An assassin snail hunts and eats pest snails, and a small group will steadily clear an outbreak. Certain loaches — yoyo and dwarf chain loaches — do the same in a larger, fish-only tank. See the full list in what eats pest snails. Combine a predator with less feeding and manual removal and the pest population collapses within a few weeks rather than bouncing back.
For the underlying cause, read will my snails take over my tank, and choose cleaner-feeding foods on our fish food hub.