I am a codeslinger, and would love to write such a search engine. Using the user feed-back idea however, would not be effective in avoiding those sites until a substantial amount of users actually report a substantial amount of abusers to the search site (could take some time, and decisions on rules : i.e. do we consider a site an abuser based on one user report or many?). Not to mention new sites are created every day, and of those that are abusers would still return in search results until they are reported.
Any other idea's anyone?