I started a thread in June of last year, than ran on for apx 3 months and covered a lot of ground, including the annoying trait that you describe; namely new members posting code that has not been Markdown formatted.
Maybe you’ve seen it, maybe not.
The link:
I’m not too sure what the answer is, or even if there is an answer, but one thing that was tried (and, so it seems, failed) was a plugin that was to detect unformatted code and alert the user.
My suggestion was possibly too authoritarian to be fair.