As someone who runs a forum where stories are posted, I can tell you that it’s not possible to split a post the way you want to. Your only option is copying and pasting into a new reply and leaving a staff notice on the story post that was edited that links to the reply.
Your best bet is user education, but I can tell you from personal experience that it will only get you so far. Some users will insist on posting those comments with the story posts no matter how much you discourage it.