blocking unwanted traffic from hitting gateway

I've got about 1000 people hammering a Linux gateway with http requests, but only about 150 of them are authenticated users for the ISP. Once someone authenticates, then I want their traffic to pass through okay. But if they're not an authenticated user, I would like to ideally block those http requests (e.g. Google updater, AV scanners, etc) from ever tying up my web server. Is there some sort of box I could put in front (e.g. OpenBSD pf in transparency mode) or maybe some sort of filter on the webserver? This solution would need to be tied into the authentication services so authenticated users hit the gateway. -- Also on LinkedIn?  Feel free to connect if you too are an open networker: scubacuda@gmail.com