If you’ve ever been caught up in linkspam, you may have seen a message in Webmaster Tools about “unnatural links” pointing to your site. We send you this message when we see evidence of paid links, link exchanges, or other link schemes that violate our quality guidelines. If you get this message, we recommend that you remove from the web as many spammy or low-quality links to your site as possible.
This can be done using Webmaster tools new Disavow links tool.
Link : webmaster tools disavow tool