The poo-in-the-hole trick worked for my dog too, and it's the trick that we most recommend in our puppy classes at my clinic. Another trick (which seems a lot of work to me) is to get little water balloons, fill them with water, put them in the holes and cover over with dirt. Then when the dog goes back to dig there the balloon bursts and scares the dog. It probably doesn't work too well for water-loving dogs though because they probably think it's a hoot!