Web Site restriction

I have a windows 2000 network and am using AD. I am making use of Group Policy as well. I also have a Microsoft Proxy Server 2.0 running on a windows 2000 server.

I need be able restrict Web access to a single site or serveral sites. This restriction can be at a user, user group, or OU level.

Can this be achieved in my current environment?

Thanks in advance.