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.
Better than BCC is using a Distribution List with restrictions on who can send to it. Helps see who else got the email, without blowing up with reply-all emails. Obviously this only works in a corporate environment where distribution lists can be restricted.