In the past, we wrote an entry named GoogleGuy Wants AdSense Spam Sites Out. Some of the AdSense spam, scraper sites, click fraud and so on were out of hand. Jenstar at WebmasterWorld posted a thread named Reporting click fraud to Google AdSense, which is a shorter version of what she blogged on under the title Reporting publisher click fraud to Google just got easier.
It is true, it got very easy to report spam. All you need to do is click on the text at the bottom right of the Google AdSense ad that reads "Ads by Google" or "Ads by Goooooogle". Then it pops open a new window, at the bottom of that window it reads "Give Google feedback on Ads by Google". By including the phrase "invalid clicks" in the comments field (to report click fraud), it will be routed to the correct team.
Above is an image of a typical Google AdSense Ad, where I highlighted the link you click on to report such activity.