On version 0.19 you can block instances so that you don’t see any posts from that instance, instead of having to block communities one by one, you still see comments from users of that instance in other posts so it isn’t completely blocking the instance.
Not that you’ll be able to see content posted to Threads from Lemmy anyway - you cannot post threads (in the Fediverse sense) from Threads (the platform), and microblog posts are generally not visible from Lemmy. So it might make sense as a matter of principle, but it’s not going to change anything for you.