Well, after we had suffered for almost 2 years, I decided enough was enough.
I simply recorded their dog when he was in full voice, barking away (I had good quality recording gear), and made sure I recorded him for at least an hour (
used both sides of a C60 tape - remember those!!). He was again in very good form, incessant barking all the time, with only minimal gaps.
This was before our kids were born by the way it was that long ago.
On a day when my wife and I had planned to go out for the afternoon and evening, and when I knew the neighbors were in at home, I simply set up my hi-fi speakers against the wall, I mean right up physically against the wall, turned the volume up, and pressed play on the tape. The tape deck I owned had a repeat/turn around option on the cassette player, and we went out for the afternoon and evening; arriving back sometime after midnight and so the barking was played back to them for over 10 hours in total.
The following day they came around to see me - quite irate about it too (cheeky devils) - and in return I threatened to (1) send in a copy of the tape to the environmental health department, and (2) play it every time we went out, until they did something about their dog.
Subsequently when they went out, they either asked one or both of their parents to come over (which stopped the dog barking), or they took the dog to their parents house.