Ads shown sitewide from keywords triggered by few popular pages

ads appearing with no relavance to the specific pages content

[kingdavid]

The site I run has many different subdomains with diverse content with all content provided by our users (it's a blog/forum like thing, I don't want to reveal any specifics).

One very popular subdomain deals with widgets, I've noticed that ads for widgets now populate over 70% of my ad-space on all subdomains relating to many totally unrelated topics.

I can't just filter out the widget sites because there are literally 100's of different ones being advertised on my site, and I'm sure the CTR is high for widgets.mysite.com.

Why is google doing this?
Is there anyway to stop it?

[td22]

Do you use section targeting? Have you tried using a new channel?

[kingdavid]

What is section targeting?

Unfortunately using a new channel is not an option as there are well over 1/4 million pages, and each page pulls the code from a database.