I have a lot, too. And I hate them. They've caused some real damage, and they've spread tremendously. I use either a screw driver to pry them up or crush into the shell, or a "coral cutter" (really a big toe-nail clipper) to cut them open - works best if you can be decisive in cutting at the base of the neck, because they will pull back into the inner reaches of the shell if you're not quick enough (which you still may be able to destroy). I've also heard that superglue can be used to shut them inside the shell, though I've not tried it.