It's impossible to disable PRs on GitHub, sadly. See https://github.com/dear-github/dear-github/issues/84
Interestingly mozilla has effectively done this here, by using a GitHub action that automatically closes any PR with a message explaining that PRs are not to be used.
It's very silly they have to do this, but at least they can I suppose.
For the actual event we are commenting on, they have disabled all features other than code hosting and PRs.