Updates soon, but for now some easy things to you do to block most users:

Add "napster.com" to your internal DNS, but don't add any A records. Don't allow your clients to use an external DNS directly.

Add the following to your Cisco PIX config. It denies access to the IP addresses of napster.com:

outbound 1 deny 208.184.216.0 255.255.255.0 0 tcp
outbound 1 deny 208.184.216.0 255.255.255.0 0 udp
outbound 1 permit 0 0 0 tcp
outbound 1 permit 0 0 0 udp
apply (inside) 1 outgoing_dest