In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon,
set DontDisplayLastUserName to 1. Or, if you use TweakUI, on the Paranoia
page, check "Clear Last User at logon".
As a security measure, this cuts both ways. It prevents users from
discovering the names of other users, especially helpful if you have renamed
the adminstrator account, and are keeping the account name secret. But most
users' account names are based on a known pattern, such that if you know the
user's real name, you know their account. And, one can learn account names
various other ways. And, if you clear the last user, how will anyone know if
someone else has attempted to log on to their workstation?