I had the same problem with ground squirrels. I used a poison made of grain treated with arsenic. It did the job. The only problem,. if you have dogs, you have to watch them to prevent them from eating the dead squirrels. I also used M-80 firecrackers. I would light the firecracker and drop it into the hole, my son would put a 2x12 about three feet long over the hole and stand on it. I still see one occasionally in the field behind my house, but haven't seen them in the yard lately.
If you use the poison grain try to keep it below ground, or cover it so the birds won't get into it. Otherwise, you may poison some of the neighborhood dogs and cats.