I have inject each one with 1ml of vinegar straight from the jug. As long as you get the body it never returns. In my uneducated beginnings I was told by my LFS that they were harmless and even a good thing. So...one of my rocks had a few on it. Killed them all during tank cycling and have only seen a couple since then. As soon as I see one it meets the syringe with vinegar.
It can be a slow process if you have lots, but very effective in my experience. I've also heard boiling water will do the same thing.
Kevin