I'm pretty sure Brad talks about this in his book. How to teach your dog when to warn you which is acceptable and when to be quiet, but I don't recall how he does it.
My girl is a barker at other dogs when we are in the car. I ended up literally having to haul over on the shoulder to pin her in the back seat and interupt her outbursts. Then it got so I could reach back and place my hand over the top of her muzzle and say NONE and now I can just say None or if she starts and it's safe, I reach around and give her a quick jab in the body or a loud and short HEY! and that seems to make her quit.
TODAY, we were out running and four cars in a row had one to two dogs in them and EVERY single dog went ballistic at the windows barking at Ella and previously this would have made her nuts. I'd have been dragging her down the street while she lunged and snarled and barked back at them but today...
she started to prance but didn't say a word and didn't even look back as we passed the front of each bumper. Man was I happy. I think she was too tired to care.

(We were working hard today.)
Do you have the book Synergy yet? It's worth having especially with yours still being a pup.