I believe it is very hard to implement algorithms that can make a difference between stackoverflow.com and a rip-off, or a legitimate Apache mailing list archive and a rip-off.
Why not allow the community to sort this out. "Google Custom Search" already exist. Google could extend that to the direction where people could customize the Google search to exclude certain sites from the results (right now it is only possible to specify a list of sites to include in the search).
Blacklists for at least specific "fields of searching" would emerge very quickly. People could select what blacklists to use, if any.
Why not allow the community to sort this out. "Google Custom Search" already exist. Google could extend that to the direction where people could customize the Google search to exclude certain sites from the results (right now it is only possible to specify a list of sites to include in the search).
Blacklists for at least specific "fields of searching" would emerge very quickly. People could select what blacklists to use, if any.