I think a "PR quality score" would go a long way here. Doesn't even have to be displayed to the user, you can just flag it as a low-quality PR under a certain threshold and have it go to a separate view for the maintainer. Have a prominent 1-click button to close it as low quality/spam with a default message about useless PRs. To go along with this you'd probably want a "report" button on comments/PRs to flag them as (spam/AI generated/useless change/etc.)

You could estimate quality with: number of PRs accepted before (only counting repos >2 years old), age of account, size of diff, number of PRs reported as spam.

Thank you for looking into this. It's a huge problem for maintainers these days... something needs to be done.