Surely the letters say the IP address of the offending downloader, not the machine used to catch them.
What about the copyright owner using a normal ISP with dynamic IP addresses allocated, firing up a p2p app and downloading from you? You're ilegally hosting the file, they're just a common ISP's user, and you can't blacklist the IP or range or you'll cut out all their customers from your p2p system.
Basically I dont think there is a way to tack on protection to current p2p apps to hide from those that can request IP->user details from ISPs.
Oh and if you read the linblock blurb you'll see it just submits some new rules to the linux kernel firewall. System which controls network connections to your machine=firewall.