Jrap, that's pretty funny, I haven't seen it before. I guess if they sue someone, they can throw this provision in along with the kitchen sink when they tally up damages.
It seems like robots.txt has occasionally been shown as valid by courts, at least as far as determining whether someone had the right to spider another site. Since robots can't read the amount they are supposed to pay, though, I think the site owner would have a lot of difficulty proving that an enforceable contract exists.
"Unwanted" is kind of nebulous, too, as is the exact meaning of "denial of service". On the other hand, if they were getting persistent abuse by a robot and they served written notice to the abuser, it might be a more interesting day in court.