The girl can simply say, "I'm-sorry, but I'm waiting for someone," if she is not interested. Such pick-ups are among the easiest to make. In fact, I am sure that girls in such situations are expecting (and probably hoping for) men to approach them. But they are ready to use the “waiting for someone" excuse if they do not fancy the man approaching them. It is really quite a painless rejection for both the girl and the man.
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It is amazing that many men think nothing of approaching a girl at a party, but would be afraid to approach a girl in a park, restaurant, or beach in the manner described.