Blocking Myspace to a subnet at a High School

Ok i know this is a touching subject but need to block myspace, linewire and all the other p2p on my site as they are hogging tons of traffic.

I have a PIX firewall in place but not to sure how to use it. It was already here when i got hired.

Any suggestions? Also i do not have a budget for any of this to work on. So this need to be an open source system solution package.

Regards

Andy