Domain in logon box

[BobClay]

With a WindowsNT4 server and a network of 50 computers in a school I am having constant calls to workstations that 'wont log in'. Inevitably because little fingers have been changing the Domain name (I have just one domain). Is there someway I can lock or hide this entry in the logon box, or make it otherwise tamper proof ? I've searched back through the records (this has to be a common problem, especially in schools) but maybe I'm not searching well as couldn't find an answer.
Bob Clay