Help! Attempted hack
 

Using 3.8.4 and have dl'ed the latest to upgrade later.
My problem is this. Yesterday I had two "users" register, using "ADMIN" and my (admin) username. Getting the email I went in and banned, added IP addies to ban list and changed the user names they were trying to use (ie mine).
Today they tried logging in, unsuccessfully but I noticed all my posts are showing as the names I gave them when I changed them in admin control panel. Initially I called them No Way and Not Available. Seeing my posts credited to those names I changed one to Test to see if it affected my posts. Yes, comes up as posted by Test.
I go to the user Test in the forum and all my details come up as though I'm looking at my own User details.
I then deleted "Test" but the forum is still showing posts from Test, which are mine. Before deleting that account I checked that there were no posts showing in admin and that account was credited with none.
What can I do to remove these accounts? How can I remove credit for posts in those accounts.
Please help, I'm stumped.