Like the way Google did with the Usenet archive. Make it available via a searchable interface, then gradually degrade the archive until old posts can't be found any more.
They also degraded the interface so that the posts that once would be found via normal searching now aren't anymore.
After the removal of the discussion filter, searches for X started returning more sites selling X than forums talking about X.
There was a thread on Google forums about that, and I recall many upset users asking for the option to be reactivated, but (the irony) that discussion was removed as well.
