spanking another child?

One of my friends on Facebook posted that if she saw someone's else's child acting up in public and their parents didn't punish them that she would grab their child and spank them for them. She deleted the post after people went off on her, but some people were agreeing with her. Now if it was me she'd catch these hands. I would straight up beat anyone's ass who touched my child like that. What are your opinions? Would you allow someone to do that or is that grounds for them to get their ass beat?