There is no one IP address you can block. The gnutella network is ran by the internet users of the world. It is not controlled by anyone.

If you wish to block access to gnutella, you are better off doing deep packet inspection filtering for "Gnutella 0.6" or whatever the connection version is... heh

Gnutella is always evolving and awhile back you could just block a port (6346) and that would stop most people who dont know how to change their port number, but now that limewire can detect port blocks and port in-use, that way of blocking can nolonger work. Soon, you wont even beable to block it at all. TLS or some other encryption method will disallow the connection to blocked all-together because you wont beable to filter an encrypted connection (you wont know a Gnutella TLS connection from an HTTPS connection). So this fix may work in the short run, but its not a long-term fix.