The key to getting a dog to stop fighting with other dogs is proper communication. The dogs need to see the humans as 100% pack leader and the communication needs to be that the humans do not approve of them fighting with other dogs. I have a Boxer that used to attack other dogs. She did this for years. When she was about 6 years old I started watching the Dog Whisperer with Cesar Millan and from there started studying more natural dog behavior. I learned how to communicate with her in a way that she could understand.
I told her that I was her leader and I did not wish for her to fight. It took a while for me to learn her body language, but when I did I was able to read her and correct her at the right moments. It's been years and she no longer goes after other dogs. Once in a while I see the look in her eyes and I simply have to give a verbal command at the right moment and she responds. The key is to learn how to read the dog, how to correct at the right moments and how to communicate with them in dog language that you are pack leader over them 100%. Tell them that fighting is against the rules. When they are convinced and so long as they are getting enough mental and physical stimulation to keep them from having bottled-up frustration, they will change and no longer pick fights. The humans need to be confident and strong-minded in order to convince the dog.