If you're you've done a good job removing the infection it's not your system sending out those SPAM emails.
One of the things that worm did was raid your system for email contacts and relationships and then cross-load that data to the spammers database. And those databases get resold and passed around. The spammers use your email identity and match it up with your contacts to allow the spam to bypass a lot of the known commercial and custom email filters since you're contacts are probably 'white listed'.
So now you have a spammer's botnet with your email data and contacts doing the actual spamming. There's probably no practical way to defend against type of activity without you changing your email addresses and having all your contacts 'blacklist' the old email address.