And everything (besides ignoring PRs) requires content to be explicitly added to the repository to configure GitHub's behavior. For some use cases -- like mirrors of repositories hosted elsewhere -- this isn't possible.
If someone's not going to bother themselves with checking the readme or contributors guide before submitting a PR, then they shouldn't be bothered by their PR being ignored or rejected.
- Owners are of no obligation to approve, close, merge, or interact with a PR in any way. If you don’t want to merge a PR, just don’t.
- you can include a readme which can include whatever discouragement you want.
- if you have a CONTRIBUTORS.md file, GitHub will show that to first time contributors when they submit a PR.
- I’m sure you could trivially have a GitHub action that will auto close all PRs as soon as they’re submitted.