Google added a '+1' feature to promote certain results. Once a few people upvote a result, it's universally seen closer to the top of the results page.
The Google Help pages include a way to report some inappropriate or totally irrelevant results but not to '-1' them.
How do I downvote a result, if it's not useful to the subject (while still a bit relevant)?
Update. This question is about results when not logged in — which is what everybody would see. Hiding results just for me isn't the matter.:)
Best Answer
The same reason Facebook won't add a dislike button; things can really get ugly if we allow people to dislike and down vote.
Take for example if all the Coke people start down voting Pepsi websites, or fans of one football team start down voting rival team's online presence. This can potentially turn into a full blown riot. Antagonizing is not the way to build a positive environment.
Plus, if you really don't like any of the results on the website, you can hit the back button and in some cases Google allows you to hide the website the next time you search for similar stuff again (as long as you are logged into Google).