A murderous hound dog worked well for us. Just make sure whatever flooring you have just inside the pet door is easy to clean.
The dog spent all its free time outside voluntarily for a while, including nights, and word must have gotten around because even when she moved back into the house no critters ventured back in the barn. She also killed the rats chewing on the car engines and the squirrels in the attic. Plus whatever was eating my bulbs in the yard. I miss that dog.
