Go to Dial Sessions>PhoneBurner Settings>DNC or Admins go to My Account>Dialer Settings> DNC and the list is found there.
You can add numbers manually, delete them individually or delete all. Deleting DNC entries does not delete the related contact. It removes the phone number from your dnc and leaves the contact in your contact manager.
If you are on a team, adding a number to the DNC list will add it to the entire teams list.
The dialer will skip numbers that are marked DNC.