How can I get my dog to stop barking?

I have a 1 year old pug and we live in an apartment. We just got construction done on ALL of the apartments and went from enclosed balconies to ones with bars. Now he sees everyone walkin by, and while he LOVES anybody he comes into contact with, he now barks at them ping by and anything unfamiliar. He was not a barker when he was younger and I would like to learn to stop his barking so we don't receive any complaints and so I do not have to keep him from the balcony. Help!