New booking (guest issues)

Hi all, I'm new to Booking.com I just had a quick question. A guest has just made a booking with me, called me to confirm, when I asked for his name he stuttered, the name on the profile is a Portuguese name but he didn't sound like he was foreign, he's new to booking.com, his phone number doesn't match the one he has on Booking.com. In simple terms I don't feel comfortable renting him my apartment. I've requested to cancel his booking but he is not accepting, what can I do, and also what can I do to prevent people who are new to booking.com to be automatically declined? I'm on airbnb also Kind Regards