In the matter of personal relationships, particularly with the opposite sex, you would be wise to avoid being too direct until you know the person well. It could easily cause offence.
However, if I was a girl being propositioned by someone I don't like, I would possibly be very direct with the full intention of getting rid of that person - "What makes you think I would go out with you in a million years? Get lost!"; if, on the other hand, I quite liked the person but am not sure about him, then maybe "Thank you, but I have a lot on at the moment. I don't have a free moment for a couple of weeks." That leaves it with him to ask again after a while.