"Hmm... that is true. It would have to be a considerable height, but not allow for them to get trapped if they do get over it."

Maybe something climbable that was only on the inside so they could get back out, but not be able to use it to get in.

"That is a good option. Perhaps there is a plant here that they do not like the smell of. If we could plant it along the fence or make the fence smell of it, they would stay away."