Filtering Spam Ads

Comment spam caused topic shift.

[Hanu]

I run a hobbyist site with some technical, IT-related content. I put AdSense on it to cover the hosting cost. The site is dynamic (Drupal) and has a comment feature so that every visitor can leave hints or questions. In order to prevent comment spam I resolved to a simple captcha check which worked until a couple of days ago when the spammers found a loophole and covered my site with spam (gambling and pharma).

The spammer also targeted ads to my site. Removing the spam was relatively easy but the spammy ads still appear on most of my article pages, even 10 days after all comment spam was wiped.

Question 1: How did they target my pages? Did they bid on a words or phrases that only occur on my pages, like typos or very specific phrases? Or is it just that they are willing to bid more than any of the legit advertisers for that topic?

Question 2: How can I filter the spammy ads without having to filter every single one of their mass generated domains?

[Hanu]

Bump. I hope this isn't a really stupid question ...