What happens if you knowingly sign for a USPS package not intended for you?

I know that the shipper and recipient are generally out of luck if mail is misdelivered, but what about a package that has to be signed for? Believe it or not an international EMS package I was expecting was delivered (and signed for) in a zip code with only 1 digit in common and totally different name, address, city etc. The only similarity was the state (FL.) This person now refuses to surrender my package. I was wondering if anyone knows if there are any federal laws pertaining to signing for a package that's not yours?