It sounds like your server is compromised, assuming you aren't the one spamming.
1. Login to WHM and go to Mail Queue Manager, and click Delete all messages in Queue to delete all emails that are going to be sent, but haven't yet.
2. See if you can find who is sending the mails by looking in /var/log/exim_mainlog, if you see it, check the user for any vulnerable scripts, or simply suspend the account so you can look at things without the risk of any mail going out.
3. Once this is all resolved, hire someone to secure your server; I can do this and there are others out there too.