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I had this problem for a while; maybe you have the same issue. Go to a followed article and look just to the left of the menu button in the top-right corner. Do you see a very faint white word saying "Followed"? Click on that to unfollow.
This is a problem with the app. I have a close-to-white background color, and white text on almost white doesn't work. I'm not sure if you can change color settings somewhere.
On the widget, you can go in and where you see the “following” flag at the top, click it. It will show all the updates to the topic. On that screen, tap the drop-down menu at the top right, and choose “clear updates.” I’m not certain that will keep away any future updates, but it will clear the notification bug, and as the story gets older, there will be less and less updates, if any.