Given how many of those SEO spam sites are themselves generated by ChatGPT now, OpenAI can simply back-reference their own logs to find out which sites are probably SEO spam while everyone else is left guessing. That's vertical integration!
It won't directly match ChatGPT logs and OpenAI would just be pouring precious compute to a bottomless pit trying to partial-match.
Serve users a random version and A/B test along the way.
I’m sure they will/are tackling this at the model level. Train them to both generate good completions while also embedding text with good performance at separating generated and human text.
I can only await companies' attempts to publish enough junk to create an 'alternative truth' for new LLMs to believe in. The worst part is, it might even work.