Citronella spray.
It is incredibly effective and humane. We have an anti-bark collar for our little dog and it works really well. My girlfriend has also used a portable version to brake up dog fights: http://www.cooldogtoys.com/directstop.php
The noise is startling, and the citronella is an overpowering smell, especially for a dog. It doesn't hurt the dog (which could make them more angry) but it startles, and confuses them.
You could spray the dog over the fence, but I think it might be better to talk to your neighbor politely and see if they'd be willing to put a citronella no-bark collar on the dog. Offer to buy it for them. This is what we use on our dog: http://www.amazon.com/Premier-Spray-Sense-Anti-Collar/dp/B0002D31QU

