I have blocked an instance in my profile settings, which seems to block posts. However the instance’s user-comments still come through and are still visible and they react to my comments.
I blocked a few users from that instance which makes the comments disappear, but how can I block all users?
Instance blocking is just blocking all communities on that instance, unfortunately.
Like Blaze said, you can try Piefed (LW also operates a Piefed instance) or you can use an alternate UI for Lemmy that does hide users from blocked instances. Not sure which do except Tesseract (https://t.lemmy.world/) since I wanted to be able to do the same thing.
The setting for that is under Settings -> Filtering -> Hide Users From Blocked Instances