Pitbulls Killing Things

Although, i consider myself a good animal trainer, I am at a loss for this one. My two pit mixes won't stop killing small animals that wonder in my yard. I live in Georgia with an usual amount of rattlesnakes. I am afraid that they will get a hold of one one day and die before i realize they've been biten. I started to monitor them closely when they are outside but I cannot watch them 24/7. I crate them when I can't watch them but I wish that they could play nicely outside. It's not just snakes it's anything else that comes in my yard. I've found birds, lizards, frogs, squirrels, armadillos dead. I always tell them "No!" when I catch them but it doesn't get through to them. I know how to prevent it from happening but i don't know how to stop the behavior. Will they ever be able to play freely in the yard?