I interplant poisonous or distasteful plants like daffodils and irises between edible ones. I also put lava rock into the bottom and sides of planting holes of susceptible plants. Running a water hose for a long time seems to help decrease damage. I had one full-size apple tree loaded with fruit fall over from the bottom, apparently with the roots eaten off, but another tree that started to lean was saved by staking it up and running a hose under it. One of those deep watering/fertilizing metal tubes that attach to a hose might be able to collapse the tunnels. My husband likes to bury dead animals in the tops of vole holes. Not having too much cover for them in low plant growth also makes it harder for them to do their surface runs and exposes them to predators.

get a cat... ours has caught prolly 12-15 this year so far.. He is the great gray vole-hunter!