Status code 451.
From the Guardian:
Tim Bray, a fan of Bradbury's writing, is recommending to the Internet Engineering Task Force, which governs such choices, that when access to a website is denied for legal reasons the user is given the status code 451.
"While we may agree on the existence of certain restrictions, we should be nervous whenever we do it; thus the reference to the dystopian vision of Fahrenheit 451 may be helpful. Also, since the internet exists in several of the many futures imagined by Bradbury, it would be nice for a tip of the hat in his direction from the net, in the year of his death."