I have a dog who loved to dig. When I'd fill the hole and re-seed, he'd just dig it up again. One day I was watching him wander around the yard, and I noticed he took extra care not to step in his droppings. So, the next time I filled up a hole, I buried a little dung under, and left some dung on top. He avoided the freshly-seeded grass, and his droppings made excellent fertilizer. This won't work for all dogs... I also have another dog who loves to dig. This trick does not work on her, as she does not care where she steps.
Please Note: the feces of dogs or any other meat-eating animal are NOT SAFE to use as fertilizer on plants that will be eaten by people, such as veggies, fruits, or herbs. The feces can spread disease, even if it comes from a healthy dog.
Is your dog digging? Try putting cayenne pepper in the holes - they don't like the sensation when they go back to dig again.