Remove second domain from Login Window

OR “Grey” it out.

Our student systems log onto an NT4 domain called the ITDOMAIN which the users see in the 3rd or lower box of the Login Window. The box continues to show all domains on the site despite the fact that the cannot Login to any other domain. I need to get the box to only show the ITDOMAIN and no others, or they cannot be selected, or the are greyed out. Users insist on trying and of course the next user is then presented with the wronf domain by deafult.