This is like trying to stop spam by banning emails that send you spam.

They can spin up LLM-backed contributors faster than you can ban them.

If the situation becomes that worse, I agree with you; otherwise, I don't see that as a problem.

Banning AI would hardly stop that, the LLM contributors would simply claim they're not AI.

Hence why banning AI contributions is meaningless, you literally only punish 'good' actors.