We have groundhogs on the grounds (under the grounds?) at work. The workers in charge of filling in the holes fill them in and the next day the holes are back. Occasionally "critter ridder" will put out a few have a heart traps and catch maybe 1% of the groundhog population.
When I was a kid I lived in a horse farm and groundhog holes were a lethal problem. What we found worked best was used kitty litter. Once that goes in nothing else wants to live there and they move out.
And I know it is kinda final solution but if you want to get rid of the problem, carbon monoxide works well for "neutralizing" the squatters before filling in the hole.