How can a site be blacklisted from an advertiser’s media buys?
Add the URL to a channel that is assigned to the advertiser’s exclusionary channel targeting
Use the audience composition report and exclude audiences that visit that site
Add the URL to the advertiser’s URL field
Identify the website’s key phrases and use them in keyword exclusion targeting
Exclusion: This is using a channel as a preset, excludable list of publishers where you don’t want your ads to serve. If you exclude a placement at the advertiser level, you cannot include it at the ad group level. You can exclude additional placements at the ad group level.
Read more here: https://support.google.com/displayvideo/answer/2717490