He has been given warnings when his opponents have beseached them to do so, but it's never been taken to the next level to my knowledge.
You are right the first umpire to actually enforce the rules, and give him a warning, point penalty, game penalty, and/or default Nadal for his delays is going to find himself in some serious hot water. He will be the subject of numerous letters from Tony Nadal threatening suit, or just to take Nadal off the tour... no more googoo eyed fans oggeling at his bulging biceps. No more 'Vamos Rafa!', no more fist pumps and gritty clay court play.
I'm sure that has quite a bit to do with his intranscience.
I think it's pretty unlikely Nadal would pull off the circuit because of something like that - there's just too much to lose (ie, millions of $$$). He couldn't make a fraction of what he does at tennis in any other career. I guess there's the argument that he's already got enough and could just retire, but ehhh, I doubt it That said, even if he were to quit, it's a real shame that they don't enforce the rules equally - it's downright dispiriting.
Who said anything about quitting?
I didn't say he would quit, I just said they would posture and threaten to walk away. When you go into negotiations do you ask for what you want? No you ask for 2 or 3 times what you want and then settle for what you want. It's about positioning and pushing and prodding. Reality has very little to do with it.
It's a distant possiblity that they (and they are Tony and Rafa, they are one in this respect) would actually do something like that. But there's nothing to say they can't make the threat, and try to see if they can strong arm the ATP. I believe they would try.