Monty,
I have lived on a lake with plenty of cmm and I now have a pond with them. Two things work. One, cats! I had two siamese that would literally go into a hole and bring them out, play with them, then kill them. Cats are too quick for snakes. Second, sulphur. If you have an area you know they cross pour sulphur on the ground. If they crawl across it they will die. I've tried this and found a dead snake a couple days later.
Of course, a shotgun is best if they'll show themselves.
