I really don't see SO as being the problem here. I happen to think that secret SEO/page rank algorithms are the culprit in many ways. Google is in the unique position as to be able to write a "how to behave" guide for the internet. Play by their rules and you are golden, start to violate them and you start to be "punished" in a gradual (but exponentially more severe). The whole time full feedback and data on what you are doing wrong is provided. As this approach evolves spammers would be less and less successful and might just have to resort to legitimate means of marketing.
Furthermore, the search engines could nip this kind of abuse in the bud, by ignoring the "nofollow" attribute of the links on a site if it determines that site is abusing it (e.g. if all or some high percentage of the links on the site have "nofollow").