If the restriction is due to legal reasons, then the appropriate HTTP error code is HTTP 451, "Unavailable due to legal reasons."
This is typically used in the case of material that has been revoked due to DMCA action or lawsuits due to harassment campaigns or the like, but the spirit and letter of the response definitionspirit and letter of the response definition states:
This document specifies a Hypertext Transfer Protocol (HTTP) status code for use when resource access is denied as a consequence of legal demands.
The code itself is a reference to Fahrenheit 451 by Ray Bradbury.