That would be something that a client would or would not allow; it's part of the user interface, not of the server. By the time the message gets submitted to the server, there is no way to determine if someone hit "reply all" or if they hit "reply" and then filled in some other "to" and "cc" and "bcc" names.
I've never heard of anyone disabling reply-all on a client, but I've never tried.
What client are you using?