There may not be a lot you can do if somebody say in china is sending out spam using your domain as the sending address.
As 2Pack said get in techie to look at the bounces, if they include the original mails the ip address can be traced and with any luck they can contact the isp and get them to stop.
The other option is to get whoever hosts your dns to remove the MX (mail exchange) records for your domain - but that means you won't get any mail to your domain, you could do this as a temporary measure until the spammer decides to use another domain.
Alternatively it may be a virus on your end thats sending out these mails, a lot of them have their own smtp engine so you won't actually be able to see what was sent