See if you can get the neighbor's cell phone number and email them a clip of their dog barking constantly.
Use technology - videotape the dog barking and post it on youtube.
Buy a very powerful flashlight and shine it into the neighbor's yard when the dog barks at night. If you haven't talked to the neighbors - and I hope you haven't, it doesn't work - take the anonymous approach by leaving a polite letter, then escalate from there. Please google 'barking dogs' and you'll be pleased to see lots of resources on how to deal with the problem. We have a dog and neighbors have commented, 'Wow! Didn't even know you had a dog.its so quiet.' Good luck to you.we can even get cooperation from other neighbors. We found it offensive that the Humane Society/Animal Control's response to our years of concern is, 'Dogs bark - that's just they do.' That's not necessarily true. It is incomprehensible that barking nuisances are not enforceable and that surrounding residents have to go through so much protocol (Pasadena City District Attorney's Office) just to maintain peace and quiet for the neighborhood as a whole. It seems that we hear our neighbor's dogs barking constantly.can't sleep, can't talk on the phone - can't even hear ourselves think. Very sorry to hear that but lucky you only have the problem during late night hours.