Voles are one of those pests that most gardeners don't know about until they have an infestation. The best way to control voles is to use traps. You don't mention what you're using as bait, but peanut butter alone or mixed with oatmeal lures these chewing rodents effectively. Continue to set traps. You'll know when the population is under control when the traps cease snapping. Dry or liquid castor oil treatments will repel voles. Snakes and owls prey upon voles. Depending on where you garden, you might consider hanging an owl nest box or adding a snake to your garden. Your local extension agent or department of natural resources personnel can advise you regarding owls and snakes that frequent your area.
You can protect individual plants or beds by putting up a fence of 1/4-inch-mesh hardware cloth buried a foot into the ground (to prevent them from tunneling underneath) and extending a foot above the ground. Keep your garden free of grassy areas and hay or leaf mulch, which provide hiding and breeding places for the voles. Vole populations peak annually in spring and fall. They also experience population spikes roughly every 4 years. Your garden could have been the site of a local cyclical population explosion, which means that the worst could be behind you.