Yeah! you can use Tags! to categorize your content and keyword density but how are you so sure that you are getting irrelevant ads for your web-page
Also... when did you notice you weren't getting relevant ads? Was it as soon as you posted your Adsense code? If so, it can take a little while before good ads are shown. But yes, like others have said, make sure that your content is about that you want the ads to be about. If for example your blog is about marketing, and you have an article about how a "brand name company" markets, then your ads might show things for that "brand name company", and not about marketing. Simply because you might state the companies name and that may trigger other ads. Does that make sense? Write more content on your keyword to help. Joel