How do I use ipfw to allow LAN access but deny Internet access?
Has been a while since I've used ipfw but running the following commands on all relevant computers should do the trick:

 sudo ipfw -f flush
 sudo ipfw add allow tcp from me to 192.168.0.0/24
 sudo ipfw add allow ip from me to 192.168.0.0/24
 sudo ipfw add deny tcp from me to any
 sudo ipfw add deny ip from me to any


To disable just run sudo ipfw flush again