How is treating one contributor fairly unfair to another contributor? If you want to add a "check this box to get your PR dumpster'd" checkbox I guess go nuts, but I'm unconvinced that's a good long-term solution. I find it easier to ask "Do I know this contributor, or did they follow the new contributor guidelines and submit a small, single-issue PR?", and if the answer is "no" then the PR gets ignored or, if I'm feeling gregarious and have the time, rejected with change requests. It's a pretty easy rubric.

Humans have been perfectly capable of generating huge volumes of trash code, and code that looks good at first glance but has tricky bugs or vulnerabilities, since long before LLMs were a thing. The only real change now is the pace at which shitty code can be ripped out. IMO the solution is just: don't accept more code than you can review and test. If that means rejecting 10x or 100x more LoC than you did five years ago, then... ok. It is more busy work, and it is annoying. But I don't think trusting contributors to self-declare LLM use is an answer to the problem. There are better ways of rate-limiting eager beavers, regardless of what tools they use.