You have a limited amount of time to assist a customer and move onto the next so spending any time whatsoever on "challenging" customers is an enormous waste of time. This reminds me of the guy who wrote about how he always buys two of everything from Amazon and the one that comes first is the one he keeps (or some other such nonsense).

I understand that you want to get something out of this experiment. A confrontation? I don't know. From what you've said ("out of fifty-odd companies in five years...") and how you reply to them you're essentially determined to try to become a problem for someone, anyone. Have you considered that this might be a mildly sadistic behavior?