I had this same exact problem.
I read through the returned email headers, and discovered that the spammer was using an earthlink account.
I sent an email to [email protected] explaining the situation and I attached the emails with intact headers, so they could see for themselves.
I got an email back the same day, telling me they had tracked down his account and terminated him.
No problems since.
Look through the return email headers and see if you can track down the source.
Good luck!
Z~