[islon]

My wifi router has an interface that shows every connected device and I can blacklist them based on their physical address.

[DCoder]

In the post I linked to, the TV in a similar situation was happily connecting to someone else's (open) WiFi network nearby. You can't really block those…

[ammar2]

Let it connect to your network and then black-hole it?

But that's assuming it doesn't try to connect elsewhere if it detects it doesn't have internet.

[nerdponx]

And this safely outside the scope of what most people know how to do with their routers.

Imagine having to take countermeasures like this to prevent things you've purchased from spying on you!