A few points.

Re Kent's comment,

I am not sure putinenv is helpful in this context. It is a DOS replacement for the kixtart & the KXRPC service. I can't see how that helps.

Re Les' comment,

Yes there are policies for freezing network setting but I believe they all apply to trying to change them through the control panel. I don't believe they apply to changing the domain in the logon box.

In Win9x this can be done but it is painful. You must leave the user no other option but to logon to a legitimate domain.
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Jack