Google just introduced a new webmaster tool in concept of Disavow Link. This tool will be needed only by those who where caught in link Spamming i.e. if you find a notification called unnatural links on webmaster, you will have to work on it. Initially you need to remove all those low quality / Spam links pointing towards your site(Note: This includes all links that violate Google's Quality Guidelines) manually. But it's obvious that you won't be able to trace each and every link that violates Google's Term and stop it, but should at least stop as much as you can.Because that would help you in preventing your sites reputation being drained further.
If you are the guy who has already reached this stage, Now it's time to use the new tool.
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A solely owned blog of RK Iyer with an intention to represent my thoughts & experience