If the money was sent via post, you may be able to complain via the postal service, too. That makes it mail fraud, which the post office can investigate -- they take a very dim view of people perpetrating mail fraud. Typically, people recommend sending the mail via priority post, or something like that, which generates a paper trail (can't remember exactly), along with a description of what the payment is for, but if you've got emails, that might serve, instead.