I looked around, and I can't seem to find any options for blocking specific IP's with your router. Since you have the router itself, your best bet is to explore on your own, but if you can't find anything, you'll have to get a new one to gain access to that kind of feature. In the mean time, when your console starts an update, just power off your device.
Only shut it off if it says "Preparing to update," once the actual update process has begun, if you shut it off (or cut off the power source) you risk bricking your Wii U entirely. Just want to make that clear.