Hi col45,
What a headache! :P I've got a few basic suggestions you might try:
First, train your dog to speak/be quiet. With a lot of problem barkers, the key is to get them trained to a command to make noise, so that they get the idea that its a good thing to do, but only when asked to do it. "Speak" and "enough" or "quiet" are good commands to have in your pocket, and you can train them easily using treats. Start out when your dog barks of his own accord, but say "good boy, speak" each time. When you've had enough, try giving the quiet command. Try giving treats each time between barks, make it fun for him.
Another thing you can try is putting him in time out when he barks for no reason. You can do this with the training to speak/be quiet, in that he goes to time out as soon as he barks after you've said "quiet". You need to find a quiet, dark place you can put him on his own, and get him there immediately. Either put him here as soon as he starts barking, or as I said, when he ignores a command. Leave him there until he calms down. The idea is that youre taking him away from potentially fun things and leaving him on his own til he's quiet again. Then he can come out. But, he goes right back to time out as soon as he barks inappropriately. This will take you being vigilant!! :P
If neither of these works for you, there are always collars you can try. Obviously shock collars are against a lot of peoples preferences, but there are things like citronella collars, which release a burst of citrus into the dogs face each time it barks (dogs dont usually like citrus, so its unpleasant but not harmful). You can try your own take by making a lemon water mix, etc, and spraying your dog in the face when he barks inappropriately, saying "enough" at the same time.
Other forum members might have more tips. My own way of training my dog not to bark at inappropriate signals was the second, time out method. I chained him up in the backyard for 10 minutes or so each time he barked at someone walking past or a noise, etc. He learned pretty quickly, so it might work for your pup. I hope you find a solution! Let us know how it goes!