I look inside these messages and find that some [censored] has been spamming using fake addresses from my domain name.

Are you sure it's spam? Most new viruses will randomize both the "to" and the "from" address in their payload emails, and they get their list by scanning the hard disk for emails. Perhaps someone who's got your email address on their hard disk is infected with a virus? Of course, there's no way to determine who.