There's no such thing as "public except for that guy in particular."

If any logged-out rando can see a thing, preventing a specific logged-in user from seeing it doesn't work and shouldn't be tried. It's ridiculous. And it's a great incentive for assholes to just make a new account.

The worst way to do it is how reddit handled it, where any asshole can have the last word, for free. You can't reply to anyone if someone in the thread blocks you. You can't even reply to your own comments in the chain, to notify people, 'hey, some asshole blocked me, I'm not just ignoring you or the argument.'