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