It appears (but is so far unconfirmed) that he is not willing to cease the systematic downvoting of persons even after "banning". If so, how would he have been willing to stop on threat of it?

This is not a difficult question. If Eugine assigns value to not getting banned then the threat of banning represents a disincentive. If the banning has already occurred that disincentive does not exist and all spite related motivations are likely to increase and any respect for the moral authority of the powers that be obliterated. What remains is the trivial inconvenience of continuing to downvote via other mechanisms.

Not actually having the power to stop a threat is a reason to use the power that you do have wisely. Were this just about influencing Eugine's voting patterns then it would be a poor decision. But such actions are made more to establish precedent and influence others.

Disclaimer: I was largely indifferent to Eugine being banned. He certainly would have been on my block list were this forum not crippled in that regard. But the parent ask a rhetorical question with a straightforward decision-theory related answer.