How do you know who is behind this spam? Good question.
You can look at the linking url and then you can email that url to Google and tell Google that the person is spamming.
For example,
Adwords Ad:
http://www.google.ca/aclk?sa=l&ai=CY...term%3Dadwords
How to contact Google: look here:
http://www.google.com/contact/