Bury your fencing. If you already have fencing put up, go ahead & dig a trench right next to the current fencing, and add ADDITIONAL fencing down about 8-10" - hardware cloth-type fencing - you can attach it to your current fence with hog rings, or even zip ties. Then bury it. Dogs don't like the feel of metal fencing when they go to scratching it. You can also do an "apron" of fencing - that is, fold back the grass around the enclosure like sod - pull it back, then put a layer of fencing (such as hardware cloth) on the ground horizontally, and attach that to the bottom of your current fencing. Again, they do not like the feel of the metal when digging. You could also go with a strand of electric wire if you have a charger or can get one. Place about 8" up, and once zapped, you won't likely have issues with it (or other critters that get zapped) again.