I have heard a few things.... not a professional here, but just willing to pass this on....
Black Snakes - I've heard black snakes (which are not poisonous) will discourage moccasins from taking over an area.
Cats - I got two cats when living at our last house. That female cat caught every type of snake you could imagine. She'd bring home 10-pound wood rats from a dairy barn, snakes, frogs, gophers... you name it, she didn't let it onto the property. Not any old cat'll do for this type of work. You want wild ones that might be a little stand-offish at first. I always go to farms and chase the meanest one I can find. Put it in a crate for a few days, feed it, pet it (welding gloves recommended), and eventually it can learn to like you okay, but keep that feral attitude that makes it such a good hunter. There are groups that adopt out feral cats. Visit
www.BarnCats.org for more on that. BTW, I prefer the females. The males always end up turning into big babies and sleeping all day (ohhh how they imitate men, haha). Make sure the cats are spayed or neutered!
Moth balls - If you are trying to keep them from coming in the house.
Gasoline and ammonia also work, but can be dangerous.
If I think of anything else, I'll visit again. That's all I've got for now. Good luck!