An answer from Adsense support one time was telling me to go to DigitalPoint forums as a place of information about the Adsense program. I thought it was hilarious. As for why you got the letter, I think I've received this now on 3 different times - each time I was clickbombed by some competition in one of my niches on my corporate account. Took measures to prevent it, and indicated same to Google.
Have you signed up for statcounter or Google Analytics? You may have to run through the report and see which IP address keeps on clicking on your ad. Block that IP address