Hammer to crack this nut
Whilst I was at working for a large consumer goods manufacturing firm after a similar incident (but not on the same scale) the CTO mandated that the "Reply all" button be removed from every email client (Lotus Notes) in the organisation.
And so it was done.
Why they didn't think of correctly ACLing who could post to a global disti list I don't know.
We then had to copy/paste recipients when an email reply had to go to more than one person. Must be wasting hundreds of hours every week across the organisation. Sigh.