Ristricted logging onto server when server is off

Hi
Thanks for the background info, this makes it easier for me to see what you need.
You need to disable your guest account if you havnt already done so.
The user is logging on with a cached version of the logon information, this is useful for when the server goes down but not in your case as you need to prevent this.
Another worrying aspect is that you allow all your users to log on using the same password. Very bad from a security aspect anyhow. It is better to change at least every 30 days. If they moan they will get used to it soon enough.
Go into group policy again and computer configuration>windows settings>security settings>local policies>security options and look for interactive logon:number of previous logons to cache - set this to 0. If the domain controller is not available and the logon info is not cached they cannot logon to domain.
Group policy can be overruled at the client end if they have admin priviledges so you may have to set this on the client pc also. But you need to test to see if it is required there. I have not used that particular policy but it should be ok just on the server. Your users dont have admin rights do they? Anyway good luck. You can set this by applying it to an OU or domain wide but if you want to target just this one pc then maybe if you put it in its own ou and apply it just there.
You need to create new policies rather than modifying the domain one and keep each policy for specific purposes so that applying them to ou's is easier, ie internet options policy, firewall policy, working hours policy etc.
Regards
Julie