IMO, the quickest solution would be a fence, but it would probably have to be about 3 or 4 ft high to really be effective. It doesn't have to be especially strong, so a plastic mesh would be fine. It does have to be something they can see, however. Except for the sighthounds (Afghans, borzoi, greyhounds, etc.), dogs are quite nearsighted.
My dogs will jump any obstacle less than 3 ft high, just as part of the fun of racing around the yard. If it's 3 ft or higher, though, they go around the obstacle.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams