Mike,
"Deny connection inline" should work with P2P traffic, in this case the "attacker" is the client on your network, a user, so be careful not to use "deny attacker inline" as it will also start blocking legit traffic. My recommendation is to test from a test PC and use the various inline blocking on simple "non atomic" stateful traffic to see if the blocking works. If it does, the P2P traffic could just be tunneling through http. Certain P2P/IM traffic uses various ports for various things such as "sign in", "chat", "video", etc, and have sub-sigs under the parent sig, be sure to select all the sigs for a particular parent sig.