You just need to tell AdSense that you are not involved in any US activities.
I guess you can directly do so by visiting link – https://www.google.com/adsense/v3/app#taxInformation
In case you want to know what activities are considered US activities, please check – http://support.google.com/adsense/bin/answer.py?hl=en&answer=18372
Please let us know if you need any more help in this regard.