Isn't it then more reasonable to have a policy that "people who submit low quality PRs will be banned"? Target the actual problem rather than an unreliable proxy of the problem.

LLM-generated code can be high quality just as human-generated code can be low quality.

Also, having a "no recourse" policy is a bit hostile to your community. There will no doubt be people who get flagged as using LLMs when they didn't and denying them even a chance to defend themselves is harsh.