I've been working through a similar problem with my dogs barking at my neighbours.
The first part of my solution was classical conditioning to change the dogs' emotional response - they were becoming extremely excited as soon as they heard a noise outside. I put stashes of cookies in every room of the house, and the split second I heard a noise outside I would rush to the nearest cookie station. The two most important things to remember about classical conditioning are:
- The treats should start as soon as the dog sees/hears the thing that it has been reacting to, and stop as soon as that thing is no longer there. You want to build a really strong association that person walking past = awesome treats, no person walking past = no awesome treats.
- The dog does not have to do anything in particular to "earn" the treats. You are trying to change a subconscious emotional response at this stage, not the dog's behaviour.
At first I would have to grab the treats and then walk over to my barking dogs and shove cookies into their mouths. Within a few days the barking had become much less frenetic and they were able to respond to their names when I called them. Now I could start the second part of the training - teaching them to sit and look at me when they heard a noise. As soon as the noise started, I would call the dogs and then only feed them once they were sitting in front of me. After a few weeks of this, I no longer need to call them - sometimes they don't respond to a noise at all, and when they do they come straight to me to sit for their cookie.
My progress has been quite slow because my neighbours are actually pretty quiet so I only get 3-4 training opportunities per day. In a busy office environment you might have many more chances to work on this. You might also be able to predict when disturbances are likely to occur, or even set them up by getting people from other offices in the same building to come visiting.
The biggest challenge is likely to be getting everyone on board to work on the problem, and to make sure that he doesn't fall back onto old habits when nobody is paying attention. If possible I would try to screen off the bottoms of the windows so that he can no longer see out (reassure everyone that it is only temporary) except by the areas where people who are willing to work on the problem sit. You could either make a rule that the person who is nearest to the dog needs to work with him each time a training opportunity arises, or set up a roster and take turns - half an hour each in the morning, and then half an hour each in the afternoon. In the short term some people may get a teeny bit less work done, but remind them that it will be much easier to concentrate once the dog is no longer sounding off every time someone walks past!