Disallow Logins

I’m looking for a command or quick method on a Windows 2000 Server environment of disallowing end-users to authenticate to the network while the network is available. For example prior to migrating from Netware 3.12 I was able to enter the command “Disable Login” at the server console. Any user login attempts would be denied after this command. But those logged in would stay logged in.
Scenario:
Domain Controller is available, domain servers are available, some users have been authenticated and using the network, I want them to continue to stay logged in, but I want to stop any other user from logging in from this point in time until a future point in time.

I want to restrict access to databases to a select group during a particular time in the day for database maintenance.

Thank you.