Don't provide a way to do so.

Just not using GitHub would probably solve most of the problems.

That b.s. It has to do with the fact that the project is not making it clear that it's not interested in PRs.

Don't have Issues. Don't have PRs. Just push your code to github. Add to the README that this is a mirror of the actual development branch that follows an internal process. The Github repo is only used as a way to publish the latest snapshot of the source.

The "going closed source" because of harassment is just an knee-jerk punitive reactionary response. It's akin to taking your ball and going home.

I don't understand why people don't just fork. Especially in the age of agentic AI. Take the code and slop it to your hearts content.

That is an orthogonal problem: popularity. What you're describing happens to proprietary software too. I have a bunch of OSS software that almost nobody cares about and I don't get bothered. The difference seems pedantic, but I bet there is something useful to learn there.