Earlier quoted context omitted.
Maybe it'd be better to just not have any blocking/muting built into such a system at all. If User A doesn't want to see posts from User B, that's fine. User A can have his client filter them out locally, prior to when they'd otherwise be displayed. Nobody else has to know this is happening. I don't think that User A should be permitted to prevent User B from replying to User A's posts, which in turn prevents User C…
The problem is when B comes across a A post and reposts it to B’s followers with a call to action to harass A. If you don’t give A the ability to avoid this by influencing A’s presence on B’s feed, A will quit the platform entirely, and B will have been rewarded (with engagement) and incentivized to stir up mobs in the future. It creates a toxic environment. Blocking is an effective speed bump to this happening!
Realistically, there is no way to prevent this.
Even if you prevent B from interacting with A via the platform itself, they can always just take a screenshot and post that. Or copy-paste the text. Or numerous other ways.