IF a man calls up claiming to be Lindsay Lohan's or Blake Lively's manager, don't believe him. A guy calling himself Roy Cohen has been phoning stores like Prada and Gucci, asking them to stay open so Lohan "can go shopping privately with her friends," and calling an event organizer to say that Lively and four friends would attend a party at the Empire Hotel. He then provided organizers with his number and names of the stars' "friends." But it's all bogus. "He doesn't rep Lindsay or Blake and no one knows who this creep is," said Lohan's rep, Leslie Sloane Zelnik.-P6