An inspired RFC from Tim Bray of Google: he proposes an additional code in the 400 (client error) series to cover content that cannot legally be made available to the client. Yes, I'm aware than 95%+ of readers have already dozed off, but bear with me.
Which 400-series code does he choose to represent this? 451. I'm not sure what the late Ray Bradbury would have thought of this, but my sneaking suspicion is that it would have tended towards approval.