Another way to look at this is as an incentive issue, rather than an algorithm issue. A large part of the incentive for spammers is Google itself, in the form of Adsense. They could easily reduce spam by having more stringent Adsense publisher guidelines, but of course they wont, because this affects their (Google's) bottom line. Basically Google's incentives are as aligned with the MFA spammers as they are with the consumer (if not more so), especially given the lack of viable alternatives (although thankfully Bing and Blekko are gradually getting there).