I use this code:

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables --flush
iptables --table nat --flush
iptables --delete-chain
iptables --table nat --delete-chain

# Set up IP FORWARDing and Masquerading
iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
iptables --append FORWARD --in-interface eth0 -j ACCEPT


To share a net connection across my network, but what I'd like to do is be able to stop the net being forwarded to an individual machine, (just temporarily), from the server.

How can I do this? I assume it is some sort of iptables command...

Unfortunately I'm not a master of iptables straight, but I've found that IPKungFu (http://www.linuxkungfu.org) makes doing stuff like this incredibly easy. Plus it sets up some good firewalling rules so that you don't leave yourself wide open while you're on the net.