|The easiest way to prevent unauthorised access to an email account is to simply change the password. Do this first, and make it secure. No family names, no pet names, no birth dates etc....|
If the emails are coming from your email address this should stop it happening again. They would need to spoof your email address in the future to do this after you change your password.
If you have a mailer worm on your pc it is a two step process. First while you have access to your email configuration page, change the password (preferably from a different pc. e.g. a friends or family members). Then run some antimalware scans.