Luke it sounds like you have been having a bugger of a time keeping your server secure. This could be a result of your recent hack, the server can be rooted. Have you reinstalled since getting hacked? have you locked the server down? have you changed all your passwords?

For this problem, I would first look at the header info on one of the original emails that was supposedly sent from your account. It will show the originating IP and other info about the server. Is it correct? If it is not correct, then the spammers are sending out forged emails and you cannot do anything about this. But, if it is correct, this means they are accessing your server and using your account for spam. If this is the case, once again you should be chatting with your webhost and get them to comb through the access logs, error logs, secure logs, mail logs, etc, plus run the usual gamet of tests against your server to ensure it is secure and not rooted.

cheers,
Luke