@tsyum think of it as if every post, comment, reply, etc, are all top level posts, just some of them include extra information indicating that in addition to being a post it's referencing what it is in reply to.

So even if you block somebody they can still reply to your post because really they are just making another post, just one with a indicator pointing back to yours.

I hope that makes sense and isn't actually just more confusing :-)

The underlying system doesn't really distinguish between posts and replies. The user interface can present it with replies attached to what they reply to, but that's really about how it displays to the user.