Deleting a user, but not setting his posts to "guest"
 

I have a very long time user with thousands of posts. He now wants to leave the forums and has asked that I cancel his account.

How can I do this, but still show all his old posts and still show them as being created by him? I know I can delete him, but then all his posts will be set as "guest". I'd like to instead replace that with his username, but then have it grayed out so you can't go to his profile.

Thanks for any suggestions.