G doesn't provide users to specify keywords maybe becuase of misuse to target high paying keywords, I have two suggestions to increase ad relevancy :
1). Allow users to specify negative keywords which are showing on their sites, Adsense should avois serving ads related to those keywords.
2). Use functionality similar to "google suggest" to present publisher to select keywords with matching site targeting,i.e. publisher will only be able to select keywords what SUGGEST thinks are logical/relevant to publishers site but publisher can't just use any keyword to target ads.