I'd try it out myself
Don't you dare, lol.
Their membership would be deleted, which means the person would lose all member privileges (see "Benefits of Registration"), also their posting total, and their title in all posts, past and future, would read "Guest". If they were to register again, they'd regain the privileges but would be starting over again as a Newbie, and they could take the same username only if it's still available. Only admins and mods can delete posts (which are backed up!).
