Google Releases Disavow Tool

The tool however comes with several warnings as expressed in the release video. Firstly, and foremost it is not for every mom and pop set-up but aimed at advanced users like rel=canonical.
Secondly, SEO’s should not jump in and variously disavow links they think could be below par. The main point of starting should be following a natural links letter from Google. In it Google may even point out a few examples of the kind of links they find to be unnatural. You may also have some links as result of previous spam SEO link building activity that you are not proud of and now wish to disassociate with.
Also, if some links are clearly not ones you would like pointing to your site then you should go ahead and disavow them. These can be malicious links such as ones from porn and gambling sites possibly a result of negative SEO.
Google also suggest the tool should be used only after trying to manually getting backlinks taken down. A take down would be better since other search engines and individuals will still see the bad links pointing to your site.
One thing to note is that Google will not automatically and immediately work on the disavow request. Firstly they reserve the right to ignore the request in the same way they may ignore a rel=canonical pointing to a 404 for instance. The request will also take a while to work as Google recrawls the sites though you can point out your disavow request in a reconsideration. Lastly, if you later change your mind then it will take a lot longer to remove the disavow and may not give the 100% link value it did.
Check the disavow tool here