Outside Dogs are not good watch dogs
I'm so sorry for the dogs, for you, for the neighbors. Especially the dogs.
Have you tried to show them how a dog is a much better protector in the house?
I never understood the logic of an outside guard dog. If a dog barks non stop outside all day, how do residents and neighbors know which barking means something and which ones don't.
A robber (or worse) could be in the house and the poor dog is just standing outside barking his head off. How can she protect the humans inside if she is excluded from the house... if she wanted to protect the house.... If the dog is in the yard 24/7, the yard is the territory to protect, not the house. Would the dog even protect inside if he doesn't see that as his territory?
I'm think there is something about this DDB's literature. Have you just tried printing out some stuff and handing it to them or leaving it on their door step. Maybe show them something from several places just in case they think DDB is a radical animal rights group or something (I don't think that). Maybe you can find something from the veterinary association.
Do ever see other neighbors try to hurt the dogs or taunt the dogs when the residents are not around. If so, video tape that and show it to the pet "parents." Perhaps they would worry about the dog and bring them in.
As a matter of fact, I just looked up an article from Veterinary Partner says what I just wrote. I went to the site and typed "Outside Dogs": Article. Two vets and one vet tech has recommended this site to me, so I'm guessing it's respected. You could probably also find a bunch of stuff from HSUS.
Are their other animal codes broken that you don't need neighbors for? Rabies? City registration. Intact license?
If you aren't able to approach this from an animal welfare perspective, then try to find another legal approach.
I'm sure the city has a noise ordinance. I don't know the details but I'm sure that continuous barking after 10:00 PM probably violates it. But once again, I'm guessing more than one neighbor has to complain. You can always approach it civilly if you so chose. Sue the neighbor for loss of enjoyment of your property (or something like that) because of the noise. Bring your proof the dogs bark late at night. They won't have much defense.
The max in small claims court here is 10K. Will you see any money if you win? Nope. Could this make them mad and instead of them bringing the dogs in , they get angry and treat the dogs worse.. or set them loose? Maybe. Could they try to hurt your property or animals out of retaliation? Maybe. Do they have anything on your that they could counter sue for or call code compliance or something out of retaliation? Ask yourself that.
Sigh. All just thoughts. No good answer.