The app I've been using (connect) has a manual filter via keywords, which might be along the lines of what you're looking for. I'm not sure if there's anything native that does it (yet) though.
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Not as of now, as far as I’m aware. You have to do it manually per instance, per magazine, and per user.
It took me a while but my /all is mostly free of the topics I don’t give two shits about. Every now and then I still have to whack-a-mole users and mags, but sadly that’s how it is.