We know that people use long tail words for Adwords to pay less. Long tail keywords are keyword terms which are less popular, less competitive and less searched for, but when taken collectively, long tail keyword phrases can be responsible for driving significant levels of website traffic. The benefit of using long tail keywords in your Search Engine Marketing is the reduced cost per lead with a higher conversion rate. Example: large green widgets for midgets - instead widgets By using the long tail of search people can instantly maximize Google Adwords return on investment (ROI), but that means less from Adsense. The question is how to avoid long tail words in Adsense ads?
Adsense reads the text on your page and attempts to match the topic as closely as possible with the ads available. In some cases it only matches the relevance of the site in "site targeted". So no way to do what your asking unless your content only refers to the more specific broad phrase. It's worth trying in a test page perhaps.