Greetings!

Since you are throwing the IP addresses of the ad sites to a non-existant server, perhaps you could redirect them to a real web server. Rig the web server so that it either:

a) ...gives them a blank ad, maybe rigging the web server to serve up a 1 pixel image no matter what specific URL is asked for. They do not notice any error messages or see any banners.

b) ...give them a popup explaining exactly why you are doing this, and giving them a pseudo anonymous web form to request that site be removed from the block. Perhaps you can even rig an "opt-out" on this popup, writing a cookie that you can ckeck for future popups.