To disable the last user logged in on Windows 7, you need to enable the DontDisplayLastName registry entry. There is a downside to enabling this. Since Windows 7 supports multiple concurrent logins, having this enabled prevents you from easily seeing who else is logged in. Probably, the easiest way to tell who else is logged is by bringing up the Task Manager and clicking on the Users tab.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
“DontDisplayLastUsername”=dword:00000001