Here is one solution:
Several Hometalkers have sworn by Juicy Fruit gum in vole holes. I can't vouch for it personally, but a few others say they have had great success with this odd solution. Here are some other suggestions:
Good luck, Teresa, and tell us if something works.
What has to be done is to eliminate their getting in in the first place. You might want to try tacking sheet metal (can be purchased at hardware store...I think they call it flashing) across the bottom of the box, extending out past the edge some several inches. That way they cannot get a grip to get in. Mesh won't work because they can grasp it. Punch holes to let water drain. You may have to cover the bottom of the entire box if they can chew to get in. And make sure their access from other boxes, tree limbs, patio furniture, etc. is eliminated too. And their access to climb up the vines! I was shocked to see how far creatures can jump when my dog goes after them. This may be one of those "you won't win" situations.
I have sprinkled Castor Oil around on the ground where they've been digging and they vacate the area.