You can now control the maximum number of credits each of your ads uses each day using the "Credit Cap" feature. You enter this number when you create your ad, or when you edit an existing ad.
For example, if you set the Credit Cap to 50 for your first ad, that ad will use no more than 50 credits per day. The number you enter is the maximum number of credits that can be used by that ad -- it is not a guarantee that the ad will use that exact amount.
Setting the Credit Cap to 0 (zero) means there is no maximum cap.