He can choose to curate anyone else on Steemit who is not related to these non grata accounts.

Or else they will be blacklisted and or downvoted?

Coercion is the practice of forcing another party to act in an involuntary manner by use of threats or force. It involves a set of various types of forceful actions that violate the free will of an individual to induce a desired response, LINK

Where's the specific written rule regarding this?

So when mmkk downvotes hivewatcher in order to coerce them towards a specific action,

That's "bad".

But when hivewatcher downvotes and or blacklists mmkk in order to coerce them towards a specific action,

That's "good"??

Where's the specific written rule regarding this?