To remove the domain drop-down list from the logon screen and force users to use their full user principal name (UPN), perform these steps:
1. Start the registry editor (regedit.exe).
2. Navigate to the HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionWinlogon registry subkey.
3. From the Edit menu, select New, DWORD value.
4. Enter a name of NoDomainUI and press Enter.
5. Double-click the new value and set it to 1. Click OK.
6. Reboot the machine.
The logon screen will no longer show a drop-down list for domains, and users will need to enter the full UPN to log on. For example, the user of an account called Ali in domain alibutt.com will enter the logon name email@example.com