Impersonated Domain

I have an internet domain name hosted by yahoo. Someone has been using this domain name to send virus infested and fraudulent emails to thousands of people. I don?t know how to stop this. Yahoo has not responded to any of my emails and I can?t find a phone number for them. I don?t have any of the original messages but know it is happening because I have been getting 10,000+ messages a day returned to fictitious email addresses at my domain. How do I stop this before I get blacklist everywhere?