The best response to a grocery store employee's concerns about your lack of sampling etiquette is probably not to grab them, apply pressure to their shoulders, and then shake them.
So the employee confronted the man because he was using his hands to sample food? At my grocery store that's pretty much all you can use. I guess at the produce section they have some pretzels that you can use to poke the pieces of fruit you want to try, but every where else is pretty much finger food. With that said, I'd like to think I wouldn't go nuts and attack any employees that came up to me to tell me otherwise. Of course, I guess I'll never know unless I'm put in that situation.
File that under don't take the last donut sample!