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I have found this as well - not "poison words" in terms of affecting ranking, but words that encourage ads to prevade a whole site and displace much higher earning ads.
However, I don't think you need to remove ads from those pages permanently.
The pervasiveness of these poison words seems to me to be due to a naturally high CTR. Ads containing these words create a natural urge for the reader to click them, and perhaps even have a high conversion. This results in a naturally low EPC but high CTR for related ads. In view of their success on some pages, the Adsense bots try them on other pages where they again get a high degree of 'success'.
The best tactic I've found is:
This way, you still derive some income from those topics without affecting the rest of the site.
That's a good idea, as a third alternative (to Marcia's ad removal and the more involved tactics I described).
I'm wondering if section targetting would work in our particular circumstance, though: we want some pages to have ads that are based on those poison words, but we don't want those ads to spread to other pages. That is, the words only become 'poison' when they start to appear on other pages.
It is my perception that ads will spread to other pages at the same site (or sometimes even to other sites within the same Adsense account) even when the keyword doesn't appear on that page. I may be wrong, though, in that the keyword does have to appear somewhere on those other pages, in which case section targetting would work.
When I first began offering AdSense, the pages I used it on were western-cowboy related topics. Later, as I expanded, additional topics became very diverse: fishing, hospitals, etc. The point is that they had no relationship to "cowboys".
Whenever the ad inventory runs low, or AdSense just hiccups, I get cowboy ads everywhere.
Its not a big problem, doesn't last long.