The only weak point Gnutella has is the Initial Connection Problem. Gnutella is indeed decentralised but at first you always need a known host to connect to.

Today these known hosts are hosts like:
router.limewire.com:6346
public1.bearshare.com:6346
connect1.gnutellanet.com:6346
connect2.gnutellanet.com:6346
connect3.gnutellanet.com:6346
connect4.gnutellanet.com:6346

So this would be a good place to start killing p2p.
I think if anyone would shut down the most known
hosts no one could connect to the network since they
have no place to start.

Clients who are already connected however arent
affected by that problem because they have already
built up a big cache of known hosts.