If the girl is very attractive, you can stop her by saying, "Excuse me, I know I don't have the right to stop you like this, but you look really great, and I just thought I'd like to stop you to express my admiration." She may act a bit coy, and even try to squirm away.

You should follow up with, "No, I really mean it. I'm quite sincere. It's not every day a man sees someone as pretty as you. In fact I really would like to meet you. My name is... What's yours?" This works best if the admiration is sincere; the girl usually knows if she really is beautiful. Then what you say is believable. If you said it to a plain girl, she would probably know you were using insincere flattery. She might go for it anyway, but personally I couldn't pull off a snow job like that.