How to restrict user access to internet

I want to know a way to restrict certain users from using the internet on client computers. I am having Windows 2000 client PCs connected to a Windows SBS 2003 Server. I was wondering if this needs to be done through Group Policy and if som then how or is there any other way? Thanks.

I dont think group policy is the best way to do such things. Usually, a security group aware Proxy server like ISA is specifically designed to do this things. When ISA is installed and configured, then you merely setup groups for allowing different Internet Access, both who and what they can do, so that some can use the Web, but others can use Web+FTP and still others have no access, etc.

So, it means that I will have to but ISA server as the server is right now running standard editions that doesnt have the ISA server. Can anyone tell me whether there is some other option which would not involve purchasing additional software.