Originally Hiri tried making it difficult for a person to use reply all. The intent was to help limit the careless reply all's that everyone does and cut down on meaningless emails.
This approach did not work because there are legitimate times when a user needs to reply all, and intentionally introducing a hurdle at these moments upsets users.
So now we have a standard reply all button, which is better. But I think you should consider taking the opposite approach. Instead of making it difficult to reply all, make it very easy to remove people from a reply all.
When a user clicks the reply all button, open the pop out window. If more than 3 people are included on the email, add some text to the top of the right side pane where all the recipients are listed, that says something like, "Do all these people need to be included on this email?" Then add an "X" button next to all the recipients in that pane to really easily remove them from the email. I've attached a marked image to help explain it better.