HI there
One thing I tried with my older lab (when she was a puppy and a digging fool; thought she was a terrier in disguise), was just to fill the holes with stones...after all, she was digging up enough of them. Eventually she got tired of digging through the rocks and now only digs in the back of an area I have designated as "Shiloh's Thicket." I was able to remove the stones and plant grass or plants as needed.
Terriers are going to be a little more single minded than a lab is, but the stone idea works.
Terese