Then connect to wifi and change DNS settings in options. But with the internet these days they can probably figure that out then. You could set a static route in your router and give the WiiU a DHCP reservation on its MAC in the router, then just set the static route in your router to go to 1.1.1.1 or something bogus for the WiiU's reservation address. Just make sure the wifi password isn't the same as your router admin password.
But will it let acces to the internet ? I mean for netflix or webnav for expemple, i would like something that block update servers but let others services running.