Forum Moderators: martinibuster
If the Google mediaBot sees primarily stuff about forum, forum software, name brand of forum software, etc., then it will serve ads related to skins and templates, etc for the forum software.
Look into your code - there may be many comment tags, alt tags, etc, that need to be deleted or changed to your site topic.
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Since I changed this I never see any unrelated ads for phpbb skins or phpbb hosting at all.
Same goes for a friend of mine with a mambo portal which has adsense on it. After he changes the /mambo directory to a more specific word the mambo ads are gone.