Yeah, I'm also in this camp. Saying we are going to deny all AI-written code wholesale and then asking them to properly attribute AI-written code is just going to incentivize not quoting AI-written code. Another thing is that when they say "can answer questions about it," it's also non-barrier, because the person will just feed the questions into a LLM. Short of an in-person interview, I don't see how this is going to keep anyone but honest users of AI out of the code base. I guess it could also be used retrospectively as a basis to ban someone.

Nothing, but nothing did before LLMs became a thing. The test lies in whether the submitter can explain the code they submitted, and more importantly, the why.

You threaten to ban any existing contributors lying about their code and you increase scrutiny of new contributors. You can go a step further and reach out to any other projects that the lying contributor works on and let their leadership know about the lying. There's plenty of ways to use social pressure that you're pretending don't exist. Not everything is a code problem. Some things are people problems and we've got about half a million years of experience with that.