I've got a robots.txt, and a script that loops to infinity. Actually, it's a useful page on the server, there's a list that can be ordered two ways, and switching from one to the other increments a parameter at the end of the invocation.
A robot has no business reading that specific page in the first place (in fact, they're disallowed to), and after a small number of loops (10 or 15), the webserver becomes very unresponsive, thus ensuring the robot writer will lose a lot of time on that page. Assuming reasonable technologies (e.g., mason), the url does not even have to look like a script...
