I've seen on a website a Google Adsense ad that was masked by a tricky image letting you think you were going to play a video - I could tell it was an Adsense ad because when I hovered over it I could see the URL it was going to open (starting with pagead2.googlesyndication.com). When you encounter such a website that you consider contains offensive ads, or that is in some way trying to trick you into clicking on the ad, you can report it and the publisher's Google Adsense account will be banned if found guilty. You can either click on the Ads by Google text that is shown in the bottom-right corner of the ad group, that will let you select an option to report the site, or visit directly Google's policy violation support page (bottom of the post).
Actually the list of violations is quite large, and you can report Google Adsense violation if the website contains: adult content, hacking/cracking content, promotions for drugs, gambling or casino-related content, violent/racial content, promotions for weapons, alcohol, tobacco and tobacco-related products. What's even more interesting is that if you consider (and can prove) the website in cause uses content that is copyrighted you can report it too
Finally, if the site specifically encourages users to click on the ads, places misleading images near ads, it's difficult to distinguish the ads from the content or that if it contains any form of spyware/malware, you can report it too.
The contact form to report the Adsense violation is here: Report a Policy Violation