How might one practically ban SEO? The moment a search engine uses information on a web page to determine relevance, the operator of the website can modify its presentation to bump up its rankings. There's plenty of room even within the strictest possible bounds of "being honest", and being the first result on the biggest search engine is valuable enough that you'll still get an underground SEO industry, legal or not.
Also, search providers know that users will get mad if they can't access popular websites, so there's no way they'll cut those websites off at a whim just for "accidental gaming", not unless they're compelled from above. And then you have the usual issues with corruptible officials deciding which companies are good and which are verboten.
From a legal standpoint, this seems far easier than banning advertising of any form. Which, if you'll remember, has (some) constitutional protections within the US. In contrast, it's a bit more difficult to claim such a thing about SEO. We regulate the activities of business all the time, and SEOing just doesn't seem expressive in the ways that "free speech" are.
From a practical standpoint, I do not have a clue. It seems as if this would just drive the worst of it overseas, where it is not possible to investigate or to prohibit effectively. I'll await the other guy's answer, maybe he has something more clever than I can come up with on a Friday at 5pm.
Also, search providers know that users will get mad if they can't access popular websites, so there's no way they'll cut those websites off at a whim just for "accidental gaming", not unless they're compelled from above. And then you have the usual issues with corruptible officials deciding which companies are good and which are verboten.