Greaser and algae, thanks for your comments.
Greaser, I used to feel that way until I realized you can't control it. I've had everything from planaria to Copepods, nematodes, hydra and the like. Once you fight the forces of evil planaria, you come to appreciate the simplicity of snails haha.
Algae, you might put a sponge over your intake. I don't find I've had the issue of them dying in the back of my canisters.
I agree once you get them in the tank its hard to get rid of them. For me the only fight is the preemptive fight, get them before they make it to the tank. I always check my plants well, especially the roots and do whatever I feel necessary like bleaching, quarantining ect to keep them out. This is why all my tanks are snail free.