How to stop receiving e-mails sent to the address that does not exist at my domain?

  1. Go to your cPanel -> Mail section -> Default Address
  2. Select the desired domain from the "Send all unrouted e-mail for:" list
  3. Select the "Discard with error to sender (at SMTP time)" option
  4. Click the "Change" button

Here you can find more detailed information on this subject.