I saw that you posted over on the dog forum I suggested, good for you and your dog! I knew some folks would chime in that cintronella and shock collars worked for them, but trust me, it is a terrible way to go. There are much, much more humane ways to deal with this issue (and from your post over on the dog forum, I realize that your dog was abused (before coming to you) and is already a very stressed animal. To add to that by zapping him or spraying him would make his situation a thousand times worse). So again, good for you for asking the question in the first place and then taking the time to learn techniques to actually help your dog by dealing with the root cause of his stress barking and not resorting to shocking or spraying him which do absolutely nothing to deal with
why he is barking. You are a thoughtful, caring person and your dog is lucky to have found a home with you!