What can I do to help my dog to stop barking at people?

You can try teaching your dog the 'Leave It' command. In this case, he is barking to tell you there is something new & unusual you need to come see. Acknowledge this, let him know you have it under control & he needs to relinquish any concerns to you by obeying your 'Leave It' & 'Quiet/Settle'. He should then be rewarded if he remains calm & quiet. Another possiblility is to Classically Condition him to view all strangers as good & friendly. You would de-sensitize him by giving him a coveted high value treat or his favorite reward whenever he sees a stranger. The reward should be given while he remains 'Below Threshold'/ non-reactive. Creating some distance between the stimuli and the dog will help him remian calm. As he improves he can get closer. It helps if he gets the treat only under these circumstances. This can be time consuming training & requires lots of patience. You will have to do 100's of repetitions especially if he is being territorial. Classically Conditioning him will help change the way he 'feels' and have lasting results as long as nothing bad occurs that would make him feel differently. His barking should be addressed ASAP before it becomes a habit. Barking is self rewarding and the longer it continues the harder it will be to correct. Negative punishments like electric collars may give short quick results but usually are not lasting especially if the dog is willful. It may also lead to aggression because they will begin to associate pain with whatever they are barking at. To understand his behavior more fully, I would read 'Barking, the Sound of a Language' by Turid Rugaas. It is a short book that's concise, well written & packed full of info. It should be read carefully or more than once to fully absorb the concepts. It is fairly inexpensive paperback available on Amazon. If he is being territorial wtih people walking by, one other thing you could try is to create a neutral zone by using some wire fencing & stake poles to create a double fence.The neutral zone should be at least a few feet wide. If it works & he improves, you can always make it smaller to give him back more yard area to play in. Hope this helps. I have meet many a pitbull mix that were more sweet tempered than a lot of unsocailized Golden Retrievers. If you are interested in more dog training advice, I would sign up for the Dunbar Academy. It is only $8/month cancelable at any time. If you go to dunbar.info/id307, I believe you can still get a free 1 month trial. It gives you access to a plethora of videos, lectures, articles and advice. Let us know how it goes,

The sound should startle your dog and cause him to stop barking. Then, call him away from the door or the window and command him to “sit”. Give him a treat and then tell him to “stay”. If he remains seated and quiet, give him treats for the next several minutes until the stranger is gone.

put a plastic bag over its head so it cant bark

Your dog barks at that neighbor guy cuz your dog has a problem with him maybe. How could the neighbor be soooo afraid of the dog if he was on the roof??? Sounds like that neighbor guy is afraid of pit bulls and no matter if that dog is doing anything wrong or not, the neighbor guy will still find an issue with the dog.... Maybe talk to the neighbor guy and convince him to actually take the time to meet this pit bull so the pit bull is no longer bothered by him and the neighbor can relax knowing hes NOT really going to get bit. If he keeps creating a problem then maybe he deserves a little nip ;) If he'd just take the time to get to know that pit bull, the pit bull could accept him and maybe even listen to him if he asked the dog to hush up...... It's tough, I had the same problem living next to a camp ground and they'd constantly call the cops, claiming my dog is making the dogs in the campground bark! I contacted the corporate office for the campground and laid it out in a polite, honest way and they understood. So, sometimes trying to work things out with the whistle blower squashes the beef and every bodies happy. If all else fails just let the pit bull free "by accident" and give him a real reason to be butt hurt, Just Kidding :)