How long have they been doing it for? How old are the dogs? How big? What kind of dogs are they?
There are a few choices here from the little info we have.
You can have the dogs be outside only when they are supervised, so that you can stop them when they try to destroy your plants.
You can put up some kind of fence around the garden. Depending on the size of the garden, this may or may not be a big deal. That thick plastic orange mesh fence that people put around construction sites may work to deter them, is relatively inexpensive, though it is kind of ugly. Another alternative is to run an electric wire about a foot off the ground around your garden. Electric fences are actually quite inexpensive and easy to install, and the bonus is that it may keep wild animals out as well.
Is there some kind of gross smelling icky stuff, like a dog repellent, that could be put near the plants? I don't know if this is possible or what would work, but maybe there is a product out there like it. I know they make one to repel cats.