You will need to create a new Administrator account and delete the Administrator account that is already known by the hacker. If your Theme is displaying the author link anywhere on the site then you will need to comment out author links. It is also recommended that you create another Administrator account that you use ONLY for logging into the website and NEVER create a post using that Admin account so that that Administrator user account is never displayed on the frontside of your website.
http://forum.ait-pro.com/forums/topic/revealing-the-admin-or-editor-user-name-and-not-knowing/