
Do you make the noise when you're visible? If so he'll learn it's not good for dogs to dig there - but only if you're around. When my dog discovered the pond I hid in the upstairs bedroom window and watched him 'unseen'. When he put a paw in the pond I shook a rattle bottle (not too loudly as he's sensitive, gently from a distance was enough for him). After 2-3 shakes he never went in again because it was the paw in the pond that caused the noise, not me.
Another thought might be to provide a sand box for digging. It might be easier to distinguish where digging was allowed if the substance to dig was different. Just some thoughts.