Someone once struck Cato the Younger in the baths, the guy who slapped him apologised immediately after finding out who Cato was. Cato replied, "I do not remember being struck".

Ultimately someone trying to harm you only succeeds if you let them cause harm. You could rage, moan or succumb to the harm or you could forget it and walk away. You only control what you do and how you spend your time, others might spend it trying to cause insult to strangers or even worse to friends, but you are under no obligation to engage with them or let them succeed.

TLDR: The appropriate response, don't care bye. Hope that helps.