How can I use Group Policy to hide the domain drop down list on the Windows Logon dialog box?

A. By default, when a user logs on to his or her Windows system, a list of domains is displayed from which the user can select which domain to use for account authentication. If you want to hide the list of domains and force the user to enter the domain as part of the username field, you can do so via a registry change, but no Group Policy setting exists by default. However, you can create the following .adm template file and import it to a Group Policy Object (GPO) to facilitate the suppression of the domain drop-down list:

CATEGORY "Logon Settings" 
KEYNAME "SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" 

    POLICY "Hide Domain UI" 
       VALUENAME "NoDomainUI" 
       VALUEON NUMERIC 1
       VALUEOFF NUMERIC 0
   END POLICY 
END CATEGORY

After you import the .adm file into a GPO (and ensure that it's visible by turning off the managed only policy view) enable the setting, as the figure shows. After the policy refreshes on the client, the domain drop-down list will no longer show on the logon page.