I have an ASUS N66U

I have configured the WAN interface to use a VPN Client to connect to a 3rd party VPN Server, so that all NAT LAN connected device traffic is routed through the 3rd party VPN server.

But if the 3rd party VPN server goes down, or the connection is otherwise lost or broken, the Asus N66U will route directly from the WAN connection using e.g. my ISP.

How can I stop my Asus N66U from routing any traffic on the WAN port if the VPN connection is down?