I think you just go to the 3 dots, open the community the post is from and then "block" but I'm new here and having a hard time as well.
I still see posts I saw the day I signed up a week ago, even though they have no user comments or any sort of engagement, even if I've already downvoted (or whatever the term here is). Most of my "all" is stuff I saw days ago and didn't like then either. I have blocked a few places (some more than once, maybe because I am on two different Lemmys?) But it's not going well.
It's been on my "front page" for days as well. I'm a Reddit migrant so maybe that's what's going on?