Please tell me what kind of sites can be banned for AdSense by Google?
For example, sites like Digg.com or Reddit.com can be not appropriate for Google's advertising program?
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The usual porn, hate, crime, scams, illegal stuff, etc.
More details can be found in their terms and conditions: https://www.google.com/adsense/localized-terms
They also hate get quick rich schemes, pyramid models, affiliate links, etc.
If i understood your question correctly, you can definitely exclude unwanted websites. Sounds like you may be an AdWords account holder who wants to exclude your advertisements from certain sites like www.reddit.com
Under Campaign main tab > Display Networks > Placement > Exclusions you can add one exclusion per line to specifically filter out the website you want to prevent your ad from displaying. Source
All AdSense participating sites are parts of the Google Display Network. You can exclude them or limit to your preferred sites in order to target your hand picked relevant audience.