How can I configure a user to logoff at a certain time?

Start User Manager for domains (start - programs - administrative tools - user manager for domains)

Double click on the user, e.g. savillj

Click the hours button

By default the user will have logon time at all hours. Each square represents one hour, click on the hour you want the user to be logged of at, e.g. 8 p.m. and then drag to when you want the user to be able to logon again. Click the disallow button. You will notice you can drag between days so you can easily disallow 6 p.m. till midnight for all days, then disallow midnight to 8 a.m. for all days if you wished

Click OK when finished, then click OK again to close the user dialog

Next open the accounts policy (Select Account from the Policies menu)

At the bottom of the dialog is a check box "Forcibly disconnect remote users from server when logon hours expire", check this and click OK