I'm using Chrome on Windows 10, and I'm able to hide comments by clicking on the little talk-bubble icon at the bottom of my story, then - when the long rectangular box of comments pops up on the right of my screen, I click the three horizontal dots at the upper right-hand corner. Then I select "manage responses." After that, a list of comments come up whose options include "hide this story on your story page."

I don't know why Medium has chosen to make the option so hard to access. Sigh.