PRs are not optional: there is no way to disable them on GitHub. I can't be sure that this is intentional, but it certainly works out well for them that this is one of many properties which make it quite difficult to migrate away from the platform.
https://docs.github.com/en/communities/moderating-comments-a...
Alternatively you can use a bot or a GitHub Action to automatically change the description and title of the pull request to something like “[PRs are not allowed and deleted automatically]”. But yeah not a perfect solution either…