What to do when somebody accuses you of copyright violation ?

If you are indeed violating copyright and are not able to defend yourself on clear grounds, it may make sense to acknowledge that this may have indeed happened (either by mistake, or deliberately), and remove the offending material that is copyrighted. However, there are many cases when the notice for copyright violation has been mistakenly sent (or when the copyright violation was done by a tool, which screwed up somewhere). If this matter was reported to Google Adsense, Adsense will send you a notice of the complaint, since Google follows the terms of the DMCA (link to article). Google's policy is:
It is our policy to respond to notices of alleged infringement that comply with the Digital Millennium Copyright Act (DMCA). For AdSense publishers, if we receive a notice or otherwise have reason to believe that your site is infringing, we may terminate your participation in the program.
However, if you believe that you have a strong case (link to article), then you can make a counter argument to the case. Follow the steps in the information at the link above, and you can actually fight against wrongful claims of copyright violation.