are you using suite promotions? I think you set the list of items through a sublist or even a saved search (on the promotion record), so if you do not want to apply the promotion for that item, just do not include it. If not, the requirement is a little tricky, if you add an item that apply for promotion but then add another (that is kind of a flag to avoid the promotion applied) you might need to script this to remove the coupon/promotions applied from the sublist on the transaction.