How can I break my dogs of a new bad behavior?

You need to have firm pack leadership so that they do not think they have to take action. Here are some tips from the pros:

”# When walking your dog, ALWAYS, keep a loose leash. Tightening the leash sends the message that something is “wrong” and that he needs to “worry”.
# If your dog is lounging towards the trigger of his frustration (i.e. another dog walking), remain calm and speak to your dog in an up-beat voice. This will make YOU relaxed and your relaxed state of mind will be noticed by your dog. Move away from the dog as quickly as you can while keeping that same tone of voice.”

“An effective way of helping your dog relax in the presence of strange dogs while on a leash, can be achieved by carrying some treats that your dog really likes (not he’s everyday kibble). These could be given to him as soon as HE spots the other dog. Stop feeding your dog once the other dog is out of sight. In time your dog will start associating a dog’s approach as something desirable. A second advantage to treating your dog while on a walk is that your dog will learn to focus on you. This is a great thing because it gives your dog something to do while confronted with the proximity of another dog. Consider walking your dog when hungry and yes, the food used for this type of work should be a part of his total caloric intake!”

“Use a Gentle Leader (GL) head halter when you are walking your dog OR a front clip harness such as the Sensible harness which you can buy locally or on line. The GL is a humane and efficient way of giving you more control.”

For barking when on leash:

“Start in a controlled environment where you have the upper hand – for instance a public park with some dog traffic. Find a distance at which your dog perceives the other dog(s) but can still easily respond to simple behaviors he does well. When he does respond, reward him with a special treat such as a ball, game of tug or high value treat. Distance helps you manage your dog’s emotional arousal – in regards to the trigger that causes the behavior you are trying to change. He can look at other dog(s) as long as he’s casual about the encounter – i.e. he can sit, do tricks, etc. Eventually with time, you can get a little closer to the other dogs. ”

“Please be advised, that the use of aversive treatments such as pulling on the dog’s leash, the use of prone collars, choke collars to correct the problem will only make matters worse, you might end up with a dog that is still leash reactive and aversive to you, who punish him (by pulling on the leash) every time he sees a dog! ”

Source

I don’t have much to add to marinelife’s very good advice, but wanted to suggest using a “clicker” for reinforcement training may be helpful. It is a small handheld device that allows you to make a “clicking” noise when your dog displays good behaviour. You can use it initially in conjunction with treat rewards and eventually by itself. You can get them in most pet stores.
I 2nd the recommendation to use a haltie (head harness). I used this with my dog (don’t need to anymore) and it gives you very good control.

@marinelife gave a very wonderful answer. I just want to add that you can have lots of treats with you and have a couple of volunteers walk past you when signaled to do so.

Also if this all does not work teach them the “leave it ” response. Do this “Victoria” training Separately: Use a piece of treat and put it on the floor and say leave it. Cover it with your hand at first. Work toward being able to put it down and say it. If he leaves it give him another treat NOT that one.This may take a week until really works. Then try it with throwing a ball or a toy they like. Have the dog on a lead. Say ok to get it and some times say “leave it” before you throw it. Treat the dog if they stay. Confirm this over time of a week or so.
Now try the “leave it” when your walker comes by. Threat.
This will take a long time but I used it to stop my two farm dogs from chasing the paper delivery car. Now when the car comes they run to me and look for their treat. Sometimes I do not have one so we rough house or have a huge pat session.
I also “saved” the ups man a couple of times :-) with “Leave it”