WHAT TO DO IF YOUR DOG REACTS
Depending on the situation, the environment, the dog's history, the owner's physical abilities, etc., how to change behavior is going to vary for everyone. That said, this general advice applies to all behaviors that most dog owners consider "bad," whether it is on-leash reactivity, stranger-directed aggression, or just bad manners in general.
Ideally, we take necessary steps to prevent the unwanted behavior to begin with, but no training or behavior modification plan is perfect. When things go wrong, R.E.A.C.T.
The closer the trigger, the harder it will be to get a response from the dog. Taking a few steps back, crossing the street, or just getting the heck out of that situation will give you and your dog a chance to calm down.
What went wrong? What could you have done differently? Taking a look at the events leading up to the reaction can help you prevent it from happening again and develop a plan for how to help your dog cope with that scenario in the future.
This is the difference between good training and bad. Bad training punishes the dog. Good training adjust to the needs of the dog. Whether you are adding distance, decreasing intensity, or shortening exposure to the person, dog, or object that triggered the reaction, make the adjustments to help your dog be successful.
Your dog did something right before barking. Maybe they looked at the dog and looked at you - even for a moment. Maybe they saw the jogger 50 yards away, but didn't bark until she was 10 yards away. You had 40 yards of good, non-barking behavior to reward. I don't care if you use a clicker or a verbal "Yes!" Good training requires good timing. Your dog may only look at you for a half-second. This is what marker/clicker training is all about. If you don't have a sound that your dog associates with the delivery of a reward, you could miss it.
I don't care that you feel a dog should work for praise. If your dog doesn't do back-flips for "good dog" and a pat on the head, it's not going to reinforce the behavior you want. Food is cheap, it's portable, and it is top of the list for most dogs. Unlike a game of tug, you can take it to a restaurant patio. If your dog thinks his kibble is the best thing ever, fantastic! Take his breakfast on your walk. If your dog only goes for cheddar cheese, work with it...and be thankful she doesn't want cash.
PUTTING IT ALL TOGETHER
I once took a client's dog on a field trip. We went to a local coffee shop to practice attention around distractions. An oddly behaving man in a hat and sunglasses started approaching us, startling the dog. When the dog reacted, *I* reacted: I immediately got up and walked 8-10 feet away, until the growling stopped, then worked on capturing and treating any attention the dog gave me. From there, we practiced looking at the man without barking (capture/treat), looking at the man and looking back at me (capture/treat), then returning to the mat at our table and practicing all of the above while stationed at his mat (capture/treat).
When a dog reacts with any form of unwanted behavior - be it bad manners or aggression - they are providing valuable information - information you can use to re-evaluate your plan and make adjustments that ensure success in the future.