Thank you! I've finally managed it — what was throwing me was you can mute an author from their profile page, but with publications you have to do it from a story, which I obviously haven't been opening.

Also, you can't do it from within a story on that publication's page, so you have to do it by searching the publication's name, not clicking on their profile and opening a story of theirs on the search results page?!

Bizarre, but oh well, at least I've got it done!