Oh, that's easy to train a dog out of.
When a dog barks at the door, he is alerting the pack to a strange noise. If the people around him stand up, yell at him, or anything like that, he assumes he's done a good job of informing you of the intruder. So the more you yell at him or give him any kind of attention, good or bad, the more reinforced the behavior becomes. If you want him to stop barking at the door, there are several steps to take. This can take a few weeks to fix, but here's what I would try:
You have control of the stimulus and the reward, so all you really need to do is expose the dog to the stimulus without rewarding his response often enough that his reaction becomes extinct.
First, ignore the barking completely, and don't even look at the dog when he barks. He will soon realize that you do not care about the intruder or the barking. DO pay attention to him and reward him (much, much) later, when he is not reacting to the doorbell. When you reward his calmer behavior, he will display it more. If you can get someone to come by your house and ring the doorbell everyday, this will work a lot faster. There are a lot more variables to be considered, and more refinements to be made, but at least give this a try. It worked for B.F. Skinner, so it ought to work for you.
