One approach is to have human testers. When a low quality site gets high rank, you investigate in detail how that happened and downrank the linking sites.
It shouldn't be that hard to find the bad network if you're systematically investigating all the time. Google has people testing search results often.
The problem is that this is fairly expensive. But quite possibly not the largest cost a search engine would have.
It shouldn't be that hard to find the bad network if you're systematically investigating all the time. Google has people testing search results often.
The problem is that this is fairly expensive. But quite possibly not the largest cost a search engine would have.