Restricted Installations Overridden

I have installed several Win2K Pro workstations and on all of them I restrict the user to not allow software installations. I do this by setting up the user as a ‘User’ and nothing else.

I also set the Local Security Policy, under ‘User Rights Assignment’ to NOT allow the user to Load or Unload Drivers. However, a user installed Netscape 7.1 under his profile. Netscape, during the installation process popped up a window that said, “You must be an Administrator to install this application. Would you like to proceed anyway?” And the user selects ‘Yes’ which it then completes the installation.

My question is, How can I absolutely prevent a user from installing software with this kind of alternative to the Administrative order set?
Thanks.