Back in the day, this was easier because these apps used specific source/destination ports. They got smart and coded the apps to search for unused ports. So it's difficult to block at the PIX. If you know the IP addresses of the Kazaa and/or Morpheus servers you can configure an outbound acl entry to deny all traffic to them.
access-list no_p2p deny ip any
access-list no_p2p deny ip any
access-list no_p2p permit ip any any
access-group no_p2p in interface inside
If you have access to the ISP prem router you can also configure NBAR. The router will look beyond tcp/udp port info and read into the payload and classify packets based on the content.
check out this link for info on how to config NBAR:
http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087cd0.html
Rich