On forums that I have administered in the past, I always had mods or other admins hide, move or archive the forums or threads to prevent things like this from occurring. Pretty straight forward, make an archive forum or section and move the items there, restrict access to the responsible parties and this doesn't affect the members post counts. This also protects you in cases of legal discovery and prevents a lot of admin time for recovery from backup if law enforcement comes knocking on your door.