Why not just have an easy button that you can click saying Do Not Allow Reply All?
I know that there are some ways you can limit reply-all availability, like in the URL linked here. But there’s a note: If recipients open this email in other mail applications except Microsoft Outlook, such as opening on web page via web mailbox, they can reply all this email.
I’m semi-tech savvy but I’m no programmer. It feels like it should be easy to do, so either I’m totally wrong or email services are really missing out on a great thing they could do.
When your recipient can “reply all”, that means you’ve exposed every recipient’s email address to all recipients.
At that point, “reply all” is just a convenience, without it they could just copy-paste the email addresses manually.
If you want to suppress that, don’t show everyone the email address of everyone else.
For internal mail, you can use BCC. For external, use a mailer service.