LimeWire will introduce an enhanced blacklist/whitelist IP-filter in version 2.5 that enables the user to specify just a few hosts or subnets which will be the only ones allowed to connect to your node. The problem is, that you'll have to get the sourcecode from www.limewire.org if you don't want to wait two or three weeks until LimeWire 2.5 is released.

Gnucleus also has a LAN version (I think).