Evil, evil snails

Hi, all;
How does one get rid of those *(%&! mystery snails that come in with
plants?!??! I thought I was rid of them, changed the filters, sorted
through all the sea weed, did three peroxide dips on the fish to get rid of
any flukes, and now, over three weeks later, a teensy snail on the side
wall! I assume it was an egg. I also assume it's not alone, even tho it's
the only one we could find. Is there anything I can put in the tank to kill
these evil things? Or am I doomed, now that they are in there?

I'm also assuming that, if I threw these plants away (they've been in the
tank since Christmas), any more I get would be apt to be infested, too....

Help!

--
Laurie, Dark Phoenix

Error. Install universe and reboot.