I don't that's possible - at least, not using the Action flags employed here (since the only options are Reply, Reply All, Forward and Reply To Folder). In any case, even if it was possible to disable this specific operation, it would be more challenging
to prevent the user from creating a new email and then dragging the original into it as an attachment.
Sorry. (The add-in is a very weak form of protection, merely a speed bump in the course of reply-all floods. To really prevent information from being leaked, I believe Exchange IRM is the way to go.)