ip_forward

On Linux, you can disable IP forwarding by doing :

echo 0 > /proc/sys/net/ipv4/ip_forward

On Windows, set the key 'IPEnableRouter' to 0 under

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameter

On Mac OS X, you can disable IP forwarding by executing the command :

sysctl -w net.inet.ip.forwarding=0

For other systems, check with your vendor.