I'd like to have some time with just you. Both people want essentially the same thing, but calling people names or blaming them is usually not productive. Note, however, that giving a reason is not the same as telling a lie. All the constructive examples so far contain reasons. Many people give up too easily after making a request or giving a rejection, especially when they encounter resistance or do not immediately achieve their goal.
Persistence pays off.
Being firm is particularly tough when doing No's. Some people will try to sweet-talk you into changing your mind ro making you feel guilty. You have to decide whose needs come first. If you are unwilling to grant a request as it is given but are willing to do something else, say so. If nothing else, the offering of alternatives can help ease any guilt you are feeling about rejecting the original request.