I encountered a similar situation in clinicals recently (I am a nursing student). I explained to the patient the purpose of NPO before surgery including the potential risks involved, notified his nurse of the situation, and wrote a very detailed nursing note explaining his resistance to the NPO order. He also had open drinks in his room and a piece of ice he'd been chewing on. I took everything out, told his nurse, and documented everything I observed.
He really didn't seem to grasp the importance of remaining NPO before his procedure even after I explained it to him, but eventually backed down.
As a nursing student, I think I covered all my bases. I'm glad he didn't push the issue any further, though!