I wonder why Google prefers that you block a whole domain instead of a result (what searchwiki did).

As said in another comment I wouldn't be confortable to block the whole expertsexchange.com domain, I get revelant results sometimes, yet some of the results of the same website are so unhelpful I'm sure I don't want to see them again (auto approved solutions and stuff like that).

It also happens when I search for something I already searched for a while back. Some of the results remain irrevelant, but not necessarily the whole domain.