My 4yo grandson does this sometimes with me. When I tell him, "The answer is no, so just stop asking," it seems to work.
I love the phrase, "Be as inevitable as the tides." If the child senses your quiet, consistent firmness, she may ask once or twice, but will probably let it go. If she senses that you are uncertain as to whether you have the right to tell her no, then she'll keep asking. It's simply human nature, and children are perceptive.
You might also be able to set the boundary before you enter the establishment. State clearly what you are going for, and that you won't be buying anything else, so there's no point in asking.
Good luck. It's completely normal behavior on the little girl's part. And your annoyance is completely normal, too. Be "inevitable."