Hi! We had the exact same problem with my 2 dogs... After 2 neighbours telling us we decided to do something about it. Mind u we had lived in the same house for 5 years with no complaints... I think its one of those things that neighbours just think they have to put up with cos theyre not sure if there's a solution and feel bad letting the owners know.
Anyway, the solution for us was to buy the best invention ever... A barking house!!! It is a plastic little house looking thing that wen turned on let's off a high pitched noise that rings really loudly in a dogs ears and is enough to snap them out of it. The human ear can hear it but it is very quiet and just sounds a bit staticcy.. It was an instant fix for us!!! Because we like our dogs guarding our house when we're not home we didn't want to stop them barking completely and have it turned on all the time so we gave it to our neighbour and asked her to turn it on for a minute when our dogs started barking at nothing. It worked instantly!! It has a long range, I think about 30 metres so u cld easily give it to a good neighbour who u trust to use it wisely and ask them to turn it on wen they hear your dog continuously barking. It's even great for ur neighbours dogs that bark all the time, ur neighbours don't even need to know!! its about $140 but really worth it, best money spent! The brand is Pet safe I'm pretty sure, look for it online or pet stores. Get the outdoor one so it can be perched on ur neighbours fence etc in the weather. Good luck!!