The first ever case of a user being banned at his or her own request?

Possibly - but who knows as others may have requested it privately. Depending on reasons it can be done yourself to do a permanent "ban" - I have done it before where a site hasn't allowed deleting of accounts just change the email address to something you don't own then change the password to a random string of characters. Essentially stops you from being able to access the account ever again.