> Some go so far as to say “AI not welcome here” find another project.
This feels extremely counterproductive and fundamentally unenforceable to me.
But it's trivially enforceable. Accept PRs from unverified contributors, look at them for inspiration if you like, but don't ever merge one. It's probably not a satisfying answer, but if you want or need to ensure your project hasn't been infected by AI generated code you need to only accept contributions from people you know and trust.
This is sad. The barrier of entry will be raised extremely high, maybe even requiring some real world personal connections to the maintainer.
Real world personal connections are how we establish trust. At some point you have to be able to trust the people you're collaborating with.