One day I told off an elderly woman who was trying to "discipline" difficult child. And, I thoroughly enjoyed myself. I, too, am tired of people who think it is their business to deal with my child, especially when I am quite obviously dealing with his behaviors myself.
Some time ago, I read a book (title and author since forgotten) by a man whose daughter had a disorder that made her unable to speak. A woman in the grocery store told him that if the daughter could not behave any better, then she should be institutionalized. The father told her, "I hope you get cancer."
I laughed until I cried. (Disclaimer: My husband is a cancer survivor and I have lost several relatives, including my father, to cancer, so I know it is no laughing matter)
Good luck. I feel your pain.