(I don’t remember how to split a reply into a new thread so feel free to split this)
Discourse supports templates to posts in each specific category. In there you could put a friendly reminder of where to go for support, etc.
I bet no one reads the guidelines, so just put a slim version of the guidelines as a comment when people do their new posts. Then people would see up front where they’re supposed to go, and if they’d have to explicitly delete it, which would mean they’d have to read it.
Also maybe a set of canned replies might be useful for this: https://meta.discourse.org/t/discourse-canned-replies/48296