If you want to block a specific AdSense ad from appearing on your site; If the ad is placement-targeted, you can now use the new AdSense interface to quickly search for and block that specific ad and future ad groups from a given advertiser.
"To search for ads in the ad review center, first navigate to the Allow & block ads tab and click into one of the pages of the ad review center -- for example "Allowed" -- for which you want to view ads. Look for the find ads box in the navigation sidebar. Here, you can search for ads based on text/url, ad type, and ad networks. For ads that are awaiting review, you can also search based on when the ads will be automatically allowed.
With recent updates we've made, you may also notice that when doing a keyword-based search, image ads in addition to text ads will be returned if the images contain text matching your search criteria," explains Google.
Try it now! Navigate to the Allow & block ads tab and click into the Ad Review Center.
[Source: AdSense blog]