If you refused to sign for a training flight that the candidate had paid for, then the term fraud comes to mind.
As many have said, the circumstances are not properly known, if however it was just a dual flight for the hour revalidation I would have made the point beforehand that I would not be signing anything if I was not happy that the pilot was safe in his normal aircraft and operation. So, lets say this was a pilot who normally flew a Pa28 from a 2,000' runway and was trying a tailwheel into a short strip for the first time as an experience, then I would be happy to sign if the basic flying was ok, if he generally flew the same aircraft into the same short strip then I might refuse to sign at the end - but I would probably not insist on payment either!