That'll plow your fields, lawn, garden or whatever too, Garg. I kinda like it, but I suspect others won't.

Run a hose to the hole and flood 'em out, if you can.

Toss one of those sulfur-contained smoke bombs down the hole. This will also let you know where the other holes are -- be sure to put a sandbag or something on the first hole, though.

But I agree with Bobert. If you can, get someone who's a good shot to sit off about fifty years and shoot the little buggers.

You can eat groundhogs, if you care to.

Blood meal will keep dogs and cats off your property; might work for other critters as well.

Or keep, as Kendall suggested, a dog.