Login to your adsense account and click on "Allow & Block" ads tabs.
However, a word of caution...if you don't know what you're doing here, better leave it unfiltered. You maybe filtering those high-paying bidders/advertisers. If you are only filtering for certain websites or advertisers you don't want to display ads on your site, this is a good place to do that.
You will be able to filter by;
More here: http://support.google.com/adsense/bin/answer.py?hl...