I would only block 4chan, patriotfront, and other stuff that most people would never know about where the cesspool is known to be malicious and full of white nationalist instigators, if anything. But then again, it would never occur to me to try to trick a family member.

If your uncle isn’t already lost, you might be able to gently talk about this stuff. There are people who discuss how to carefully offload alt-right beliefs by asking questions that reveal fallacy rather than beating someone over the head with why they’re wrong. You can find better info than I have by searching the web.

My own mother, for example, voted for Trump three times. That relationship is over, sadly. Some people can’t be reached.

Moral: don’t do underhanded shit. Be open about your disagreement, but first learn what approaches have the greatest chance of success.