When he got frustrated John Hargrave drew a picture of Shamu instead of signing his name on a credit card receipt. The cashier accepted this without even blinking. Surprised, he decided to find out what it would take to have someone refuse to accept his credit card becuase of a mismatched signature.
http://www.zug.com/pranks/credit_card/
It get's pretty ridiculous towards the end, but he finally does manage to get someone to refuse to accept his card.