A good cat is the best solution.
Failing that, snap traps work, but they need to be be pre-baited. Voles are wary of new things in their environment so you need to put out baited traps, but DON'T set them (peanut butter mixed with oatmeal is a good bait). After a few days, they'll get used to the traps and start eating the bait. Then you can re-bait the traps and actually set them. One question, have they ignored the traps so far, or have they triggered them but escaped? If they've triggered the traps and escaped, they are going to be extremely difficult to catch in the future.
