It seems like a obvious answer, but why not just use "-site:annoyingpage.com" in you search? In fact "-TotallyUnRelated" has helped me narrow down searches effectively too. You are asking for a feature that only a small subset of the users will benefit from and use, it makes more sense for google just to find a way to rank sites better then it does to build a additional filter on top of the current system.
Would you want to type out a string of 20 or 30 excluded sites every time you search Google?
Ranking clearly isn't going to be good enough, because algorithms can be worked around and gamed.