Well, if there has to be graffiti, then let there be graffiti of famous science fiction authors.

This is letter from Isaac Asimov in response to a librarian who asked authors to write to the local children encouraging them to use the new library. Letters of Note (which is generally a fascinating and wonderful blog) has more letters from other authors here.

Three Laws of Robotics

A robot may not injure a human being or, through inaction, allow a human being to come to harm.

A robot must obey orders given to it by human beings, except where such orders would conflict with the First Law.

A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.

(Just in case I ever need to check.)