I've been using the AdSense "Competitive Ad Filter" to filter out some advertisers from my website. It's ok for not displaying ads from unrelated sites that happen to match good keywords.

What I haven't seen much info about though, is what kind of control advertisers have over which websites their ads are to appear. I'm sure there's a good deal of it. I seem to be getting a lot of ads for hosting forums lately. Can anyone share some experience from the advertiser's side? Thanks for any info.

I found a good deal of info about advertiser ad management here:

https://adwords.google.com/support/bin/topic.py?topic=25