The gist is: no one will read the messages. You can edit all the existing messages, the discobot narrative, put banners up; adding a pop-up text isn’t the answer to changing community behavior.
So we might want to take another approach to this. I believe there are editing time limits, perhaps those can be configured to help.
Also, you could make it explicit that discussions are public and remain so; otherwise, it seems appropriate for a person to be able to delete their messages. They aren’t using the site the same way as you, so you ought to explain your expectations. 