I wonder why the moderator can agree to delete his threads when they will not clean a report of old posts.
Old post: We generally do not moderate posts this old for on-topicness, substantiveness, multi-posting, and other "cleanliness" issues.
As for "hide traces of past plagiarism", I'm not sure if "hide" is correct terms because isn't it fair for people to delete or edit their posts to correct their past mistakes?
I think there's nothing wrong to do that, but deleting people posts for a sake of own satisfy is childish.