There's technically a way[1], but you'd have to do it every 6 months which is not great.
https://docs.github.com/en/communities/moderating-comments-a...
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…