I've only used Gnotella, but I assume that all of the Gnutella-based apps have a similar option.

With Gnotella, under the setup menu, there is an option for private networks. You can define your own network, from there you can only access files from PCs with the same network name and only people with the same network name can access your files.

All the Gnutella-apps are free, why not just d/l the installation file and pass it on to everybody at your LAN party (or better yet, have them d/l a servent on their own before the party).