Report AdSense Spam & Click Fraud from Search Engine Roundtable has a nice, easy to understand illustration on how to report to Google if you come across a spam site that is funded by carrying AdSense ads. It picks up from the Matt Cutts announces new AdSense spam report feature and Reporting publisher click fraud to Google just got easier posts at JenSense, which provide more details on how Google is encouraging people to report suspected click fraud or spam relating to AdSense. But, Google could make things a lot easier.
Basically, the instructions are to click on the "Ads By Google" or "Ads by Goooooogle" links you see shown as part of ads. That brings up a page asking for feedback. You then have to make sure you include either the term "spamreport" or "invalid clicks" as part of the comments you send. It would be better if these were options associated with the form. Rather than having to remember what code word to employ, just give us a drop-down box or check boxes that list the problems we might have with the ads.