[CentOS] Edit login user list on CentOS 6

The thread on the CentOS 7.1 user login screen reminded me of a small nagging issue I have on CentOS 6. We are using a Windows AD backend to authenticate users on our CentOS 6 systems. When a system is built, and nobody has yet logged into it, you have to enter your username in the login screen to log in. The next time you are the the login screen it lists all the users who have logged in (via the graphical login screen). This works great if this is your own desktop and nobody ever logs into it, but on a shared system the list can get very long. Does anyone know where this lists is kept? There are some systems where I want to edit the user list, but I have not been able to find it.

Thanks, Alfred