By accident, I discovered that my garden bed with a rock border is protected by the voles. The rocks sit in a trench that goes around the border, so part of the rock is below the soil surface. Grass is kept very short next to the rocks, so they have no places to hide. My guess is that when they are digging their tunnels, they hit a rock and go in another direction.
Because I like to use organic mulches on my garden to add nutrients to the soil, the levels can quickly pile up if I'm not paying attention. Don't use more than 2"-3" of mulch on any garden bed and frequently fluff the mulch with a fork or rake to discourage them from nesting in it. Once a week should be sufficient if you have high activity, less often if voles are not much of a problem. Fluffing the mulch also keeps it from compacting which can reduce water from getting to the roots below, so it's a good idea to do that anyway. You will also want to keep mulch away from crowns of plants where rodents can hide and dine undetected.
In winter, voles have been known to eat the bark from young trees, so it's important to protect them with a tree wrap of some kind. Just make sure you remove the wrap in the spring so that it doesn't inhibit the growth of the tree.
You can also trap voles to help reduce their populations quickly, but this can be harder than it sounds. A standard mouse trap, bated with peanut butter should be placed at the entrance of their runs. I've found that using live traps like the TinCat works better, but they are still alive once in the trap, so you have to go to the trouble of killing them after you've caught them, not something I particularly like doing myself. Choose pleasant days during October or November to bait your traps when they are more likely to be active. Once you've trapped in one area, destroy the burrows to discourage new voles from moving in.
If you garden for wildlife, you may find that your population is well controlled by nature. Hawks, owls, skunks, fox, and snakes will all eat voles and the neighborhood cats will hunt them. Mother Nature can usually take care of herself without our involvement, but if she needs a little help, choose the least offensive method of control first and go from there. I've never had a problem with voles damaging an established plant in the garden, it's the potted ones that are still young that are the most vulnerable.
More information on voles can be found here.