Move off of GitHub. It's clear that neither the founders nor the new Microsoft management care about this kind of noxious spam. Or at least, they can't be bothered to give you the tools you need to deal with it (I want to be able to, at my discretion, (1) disable creation of issues and pull requests (2) force an approval process where I must approve issues and PRs before they even appear on the page (3) be able to decline PRs without the original poster or anyone else from getting an alert).
The fact that they won't add these features isn't surprising, GitHub is designed to be a corporate product, for corporate dev teams. That's who pays the bills, not open-source projects which are less obviously a benefit to GitHub (they are, they bring corporate users/payers to the platform).
It's clear you don't want to be confrontational in any way. I understand, it's also not in my nature. You shouldn't be forced to deal with this, yet on GitHub, you will be.