Fit a video camera to your helmet and start filing assault charges.
If someone beats you up and "nothing can be done" then you need to talk to some cops who will listen, do you have cycle patrol cops. Challenge the professionalism of the cops for not acting, they hate that.

If someone starts following you, double back and you force him to stop if he wants some action. Go slow enough so you are not running away. You have siezed the initiative and he is now reacting. Then force him to walk a distance from his vehicle, maybe 50m before he can address you. You have eliminated a "self defence" as a reason to beat on you
Then keep a shackle lock handy on the top of your rack. I was taught to strike the lower leg from knee to ankle in a low passing strike. Aim high as a diversion but duck and hit low. Use the bike as a fence if you want to be more defensive, this has worked for me once.

Open carry may bring out the southern gentleman in passing motorists.