No sternly worded letter, no slap on the wrist... Consequences!
I'd even "scare them incorrupt" by threatening to nationalize the company if no sufficient corrective actions are implemented to stop being a net negative to society.
Now that's the way to discipline the shareholders, which is a different problem than disciplining the leaders.
The "401k objection" is often used as cover by the defenders of those people, in a ploy to avoid accountability: "shares are held by all kinds of common people in their 401ks, and all you be doing is punishing those innocents!" My answer to that is to require public companies (with brokerages) to maintain a registry of the beneficial owners of their shares, then just nationalize the shares of the largest shareholders (as those are the people/organization with any real influence).
As for predictable objections of "it's impossible to collect that info1!1" they're already 90% of the way there, because they already know where to mail proxy statements. I think you can get the rest of the way with extra reporting requirements and onerous penalties for non-compliance (e.g. forfeiting the shares after a suitable grace period or if there is evidence of evasion).
Maybe if regulators grew a sack and started absolutely deleting companies for wrongdoing, people would think twice before they invest in them or the various institutional funds that invest in them. Wishful thinking.
Out of curiosity, if one were to "steelman" my proposal... If nationalization of companies were a consequence of public companies behaving badly, would it not pressure shareholders to divest from bad actors, rather than doubling down on those whose leaders lacking moral scruples?
What I'm trying to ask is: wouldn't it result in a better (moderately regulated) "free market" if there were consequences to leaders and shareholders alike?