how i stopped my dog from digging was simple: i trained him to dig on cue then, as we went for our walks, i would give him the cue to "dig! dig! dig!" and let him tear up the beach, the gravel path, the snow bank, the ditch, whatever (we lived in the middle of the bush, so... whatever!). i also provided him with a patch in the back yard handy to his zip line that he could dig in -it was the only patch liberally scented with cow manure (his favourite).
i guess once he learned that he'd be allowed to dig his little heart out every day, he didn't need to do it so much.