You shouldn't really do anything at this level. Obeying the law is the best option. Under the law they do have the right to say "Do you want to buy a watch?" and "Boys, jump this guy!"
You have all the right to ignore them by not answering them. And when you ignore them and they start to invade your privacy physically like trying to touch you or throw stuff at you, defend yourself first and you must call the cops to report the incident. When you don't report the incident, a supposed assault with you being the victim will turn into a fight between you and him.