You can lock the settings in Office 2003 applications with the Office GPO's. The specific GPO for Outlook 2003 is Outlk11.adm
For example, all user-specific policy settings are stored in the HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0 subkey, which mirrors most of the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0 subkey. Computer-specific policies are stored in the HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Office\11.0 subkey. By default, both Policy subkeys are locked, making them inaccessible to users.
Information is available here for the Office 2003 GPOs:
http://office.microsoft.com/en-us/assistance/HA011402401033.aspx
With Office 2000, you will need to use the System Policy Editor. The specific template for Outlook 2000 is Outlk9.adm. Information on those templates can be found here:
http://www.microsoft.com/resources/documentation/office/2000/all/reskit/en-us/95ct_10.mspx