what type of dog is it?? usually you need to catch them digging and rub thier face in it immediately otherwise they dont understand what theyve done wrong
it depends on the dog too, my parents have had 2 boxers, the previous one they did what i mentioned above and she never dug again, the current one theyve had to fence off thier lawn!
usually its boredom so make sure your dog has plenty of toys and gets walked lots
GL