Ok, let's try something a little more technical (sorry, I don't know your level of experience). We're going to delete the Messages preference file that contains any previous account info.
1) Completely closing out Messages (either by Cmd + Q or up by the Apple menu click Messages > Quit Messages.
2) Open up Finder and browse to your home folder (where you see Downloads, Documents, Music, Pictures, etc. and right-click in that white area and select "Show View Options." Check the box that says, "Show Library Folder" and then click the little red x to close out that small window. Now you should see a Library folder. Double-click on that to open it.
3) Look for a folder called "Preferences" and open it.
4) Browse for a file very specifically called "com.apple.imservice.ids.iMessage.plist"
Right-click on this and select "Move to Trash."
5) Close out this Finder window and then empty your trash.
6) Reopen Messages and try your credentials again.