<<I haven't seen an ad in a long time. Why don't you just find out the server of the ad itself (right click on the ad, click Properties, and see what the URL is)? My guess is they're all coming from one or 2 main servers.>>
The ads share a common URL but are load balanced off multiple servers. I'm banning by IP and I have about 20 IPs in my list and I only have about half the ad servers blocked.