However even if we assume that detecting spam content is hard, detecting the funding of said content is much easier - most of this content is there to get you to buy something or look at ads, and they use analytics tools to track conversion.
Use the presence of ads, analytics, affiliate links and other marketing tools as a proxy for "spam" and downrank them, or offer the user a "non-commercial" option like Kagi does.
Now you move the problem from old-school SEO "make my content rank higher" to "conceal my ads/analytics/etc from the search crawler so I don't get downranked". That is a very effective strategy because advertising has to be served via third-party code to protect against ad-fraud, but third-party scripts are trivially detectable.
Now the spammers and their partners have a problem - the only way to fool the crawler is to self-host the ads, but doing that would expose the advertisers to ad fraud. You're pitting the spammers against each other in a game where the only winning move is not to play and the extra overhead will make the spam less profitable (or not profitable at all).