They Stole My Email Address

One of the addresses I have is being used by some spammer as a hidden return address and I keep getting "Returned Mail: user unknown" messages from it. They are not using the account to send them, so changing the password has no effect. I don't know what to do about it. It's not my main address and I can just get a new one but I don't want to. I would rather find out who is doing it and make him eat his computer. The messages don't even show my address any where on them.