If you go to For Publishers -> Manage Ad Zones, click on "More" next to the site in question and then "Ad statistics" it will show you what the earnings of each ad are. Simply reject the cheapest ones and they'll not be shown again. If your site is well established with AdBrite it will probably display lots of different ads, so this approach could be time consuming!
or you can simply set a pricing option as i did .. and i currently have around 0,11-0,13$ CPM at a 30% fill rate