The next time it happens, ask the officer for his name and badge number and file a complaint with his office. The next time after that, same thing. The next time after that, same thing. At that point you can reasonably think about talking to a an attorney about filing a lawsuit, with enough paper to back your allegations.
Nothing you can do except get ID from the coppers that stop you and at some point contact a civil rights attorney. You can't fight ignorance any more than I can deny that I have blue eyes
I have a lot of respect for you being able to keep your cool during these encounters. I would become a screaming nut and then give the cops something to arrest me for.
In addition to my colleagues' good suggestions, I would also suggest that you keep a record of these encounters. It can be as simple as putting the information on your cellphone "note" or "notebook" app after the cops drive away. The more encounters you can recollect (with the info my colleagues have suggested you gather, such as badge number, etc) the more persuasive your case.