Phoney user?

On our site http://www.peteranne.it we have one user, admin, and don’t let anyone else register. Navigating the dashboard, I received the message “you are logged on as peteranne, you can´t see the stats”. Now, peteranne AT peteranne.it is admin’s email, but this is a weird message. peteranne is not listed as a user, and we had some not very bad spam URLs hacked into our site, see http://ww.wp.xz.cn/support/topic/hacked-random-links-to-other-sites-at-foot-of-some-of-our-pages?replies=3

Could this phoney user be related to the hack? How do we remove it, please? thanks Peter