Today while using Yahoo they provided this option (named "block" by Yahoo). Good. Using Yahoo Search I can now block site that hide content behind pay, or registration, walls and spam sites. Obviously it would be better if they blocked the spam sites themselves but this is a useful feature for those that sneak through.
I would also prefer if Yahoo would let me block all pages that don't display the content (that content that prompted Yahoo to suggest the link for the terms entered) without going through some paid or registration wall. But this block feature is useful in the case that they don't do so. They seem to be starting down that path (looking on the preference page but I still get many sites that are returned as matches that don't go directly to the content that was matched against).