I'm afraid, it is not possible to exclude placements at the account level through the API. However, you will be able to exclude campaigns at the campaign level using the
NegativeCampaignCriterion and at the Ad group level using the
NegativeAdGroupCriterion where you will have to specify the criterion type as required. Please refer to
this article on how to exclude placements at the account level using the AdWords UI.