Is there no physical address associated with the account? Buy a stamp and send a letter to said address saying you have their paypal email address (obviously don't include the email address or at least show just a few characters) and ask them to reply to the email address which is associated to the paypal account. If it's a fake address then whoever gets the letter won't be able to email their reply.
Give it a few months for them to reply, and if no reply appears then change the email address associated with the paypal account to a disposable address and remove the original address.
Biting the hand that feeds IT
