First of all, I'm not ruling out the possibility of purchasing a proxy appliance if that's the best solution but I would like to figure out if there's any way to block certain users from some websites.
I'm using Active Directory already so I'm thinking I'd have one or more groups. Members of each group would have a list of blocked URLs. That way they'd be blocked regardless of which computer they were using and I could maintain a separate list for each group. Is this even possible?