If you want to stop them accessing the internet then it's probably easier to configure DHCP to give them an IP address and subnet mask but no default gateway. That way they will be able to access everything on the local network but nothing beyond.
If you do want to do it with iptables, it is possible - consult the
iptables man page for details. You'll be wanting something like
Code:
iptables -A mychain -s xxx.xxx.xxx.xxx/yy -j DROP
Where x is the address of your network and y is the subnet mask, either as a CIDR or standard mask.
