When a newbie learned that he has to sign his comments in every message, he will try to add a signature in his Flow posts.
So having an alert that is triaged when ~~~~ are present in a post that is about to be sent can be great, both in wikitext and visual editor modes.
The message can contain something like

You are about to post a message in a Flow board. This page type automatically shows your signature at the top of your post, so you don't have to add ~~~~ in it. Do you really want to send your message with this redundancy ?