Yeah, that's actually what we've done on the Zig GitHub repository. However, it doesn't stop pushes to existing PRs, which isn't ideal; and, yes, it's quite hard to escape the conclusion that there being no "until I turn it back on" option is intentional.
It's completely intentional, and goes back to when GitHub was founded. GitHub was intended as a collaborative software development platform, not "look but don't touch".
You can close them and limit discussion to contributors I guess? Not ideal but at least they wouldn’t appear in the pull requests tab.
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…