The problem here isn't the users; the problem is an e-mail server that allows "reply all" to have this effect. Modify the server to prevent an employee from sending one e-mail to more than X other employees. You will always have dumb users, but your e-mail policy can be smart.
This has been a huge problem across state and federal agencies for quite some time.
One day, they'll discover BCC may be applicable for some of these emails.