bogus 6to4 router at IETF WLAN

Someone's Windows box with lladdr fe80::204:23ff:fe7a:fb3e
(2002:da25:e0b0::da25:e0b0) is advertising has gotten too smart, and
is advertising the default route on the IETF59 WLAN.

Stop immediately.

(Perhaps misbehaving hosts should get MAC address blacklisted for a 
while..?)