It should be coming soon, support for it is almost in Lemmy, once that's released and servers are updated so I can test it I'll add it ASAP! This is the Lemmy PR I'm waiting on.
I came across this by searching about blocking instances. Would this allow you to choose whether to block posts or users or only both? I ask because I wouldn't mind seeing comments from people on lemmynsfw (for example), but I just don't want to see the posts.
The PR says "Analogous to the existing community block functionality, users can also block instances. This means that all content from communities which are hosted there is hidden. Posts from users of blocked instances are still visible in other places." So it sounds like what you're looking for.
I tried switching to an open source app and was about ready to give up on Lemmy because I couldn't block the extremist propaganda from Hexbear and Lemmygrad.