I used Google to find this (emphasis mine):
If you've found a site violating the AdSense program policies, please
let us know. The information you provide will be forwarded to a
specialist for further review.
Some issues require specific information for an investigation to be
completed. Those issues include:
Copyrighted content: Distribution of my copyrighted content without my
permission
Unauthorized ad code on my site: Ad code was placed on my
site without my permission.
Ad complaints: Send feedback about a
specific Google ad displayed on a website.
If the violation you've noticed is related to AdSense, but doesn't fall into one of these
categories, then please use our violation report form. Alternatively,
you can report a site through the "Ads by Google" or "AdChoices" logo
which is displayed on each ad unit. Simply click "Ads by Google" or
"AdChoices" on the site you'd like to report, and then submit your
feedback on the resulting page.