The reason you can't open another dispute, is because they have already investigated this case, and have decided to not refund you. Due to the nature of PayPal, you are less likely to win cases regarding unauthorised authority, and therefore should try the other option. It checks your IP, and how you sent it, and obviously, PayPal found nothing suspicious. You unfortunately can't do anything, unless you contact your bank/credit card provider. Otherwise, you can call local authorities and tell them to investigate.