Fence fighting is a common problem. I've tried a few of the ultra-sonic sound machines. Good in concept, but not all that useful in practice. The best way I've used them was to set them with the human sound on; that seems to help more since it bothers the neighbors.There are dozens of ultra sonic, battery powered whistles on the market. They are so high pitched that humans can't hear them, but dogs can and don't like it. They are advertised as stopping bad behavior and not only work, but you remain out of the picture.
I've tried several methods to deter such conduct, the best one so far is a visual barrier along the base of the fence, along with about a 12 inch obstruction, or basically a fence within a fence.
After contacting a few dog trainers, training a dog to stop fence fighting is a long and arduous task, that requires the neighbors help. So, all but one, said to put up an electric fence as a last resort. Throwing tranquilizers in food, BB guns, water cannons, etc, was naturally not recommended, and is likely against the law as animal cruelty.