They already are against the rules here.
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
(This is a broader restriction than the one you're looking for).
It's important to understand that not all of the rules of HN are on the Guidelines page. We're a common law system; think of the Guidelines as something akin to a constitution. Dan and Tom's moderation comments form the "judicial precedent" of the site; you'll find things in there like "no Internet psychiatric diagnosis" and "not owing $publicfigure anything but owing this community more" and "no nationalist flamewar" and "no hijacking other people's Show HN threads to promote your own thing". None of those are on the Guidelines page either, but they're definitely in the guidelines here.
This is the correct answer. If you're curious about what other sorts of things are disallowed by common law, look at dang and tomhow's comments that say "please don't":
dang: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
tomhow: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
Thanks for a lot of references!
One comment stands out to me:
> Whether to add it to the formal guidelines (https://news.ycombinator.com/newsguidelines.html) is a different question, of course. I'm reluctant to do that, partly because it arguably follows from what's there, partly because this is still a pretty fuzzy area that is rapidly evolving, and partly because the community is already handling this issue pretty well.
I guess me raising this question is because it feels maybe slightly off that people can't really know about this unwritten rule until they break it or see someone else break it and people tell them why. It is true that the community seems to handle it with downvotes, but it might not be clear enough why something gets downvoted, people can't see the intent. And it also seems like an inefficient way of communicating community norms, by telling users about them once they've broken them.
Being upfront with what rules and norms to follow, like the guidelines already do for most things, feels more honest and welcoming for others to join in on discussions.