Admin:
So go forth my children and run your DNS servers, make them public, and block the SPAM!
And so it shall be! I've done exactly this and taken your idea a bit further. Simply null routing ad networks might 'hang up' or distort poorly design pages. I've set the blocked domains to a custom written http server that returns blank html/js files or 1x1 imgs depending on the requested content. This way the browser still receives a response from the ad, just happens to be blank.
Alternate DNS