Github Issue: https://github.com/OfficeDev/office-js/issues/1779#issuecomment-829159262
Essentially, there is no way to stop OWA from appending the original message to the body when replying to an email, unless the user clicks the "..." at the bottom of the compose window. The original message gets added AFTER any "on-send" functions defined in an add-in are ran, so there is no way at all to control the entire body of a reply message.
This is a big issue when an add-in needs to control the entire message body. For example, an add-in that does encryption.