I'm not ashamed to say I would punch the guy. Any unwanted physical contact by another is considered assault by the law. You also have to understand the human reflex to even the slightest pain, especially unexpected. You have little control sometimes how you react, the mind takes over.
Example, I was helping out with Bible School one week. I was helping out with five year olds I think. This little boy comes up and taps me on the back. I turn around and he punches me right in the family jewels. Instinctively, I sort of slapped the top of his head with my open palm, very lightly. It was just a reflex. It wouldn't have been a problem but I was wearing a big class ring and it was turned backwards on my hand. It almost knocked the kid out. I felt terrible but I literally had no control.