When I was first told this I didn't believe it but it worked. Knock all the dirt from his mound off to the side leave the hole open. They can't stand having their front door open. Get a chair and your choice of weapon. I used .410 with birdshot. Sit on him like a deer. When things quiet down, he'll come up and start flipping dirt back out to cover up the open hole.. Walk up and blast him. Done.
BTW this was after failing with all the above methods. My wife started calling me Carl. I tried everything short of plastic explosive shaped squirrels.