Airline cancelled flight and no refund given!

I booked and paid for a Fly Africa flight from Windhoek to Johannesburg. A few days before the flight Fly Africa cancelled the flight due to the airline not having the regulatory approval to fly to Namibia. I was informed via email that the full fare would be refunded within 5 - 7 working days and I was given a voucher for the inconvenience. To date (more than 10 working days later and after countless futile calls to the Fly Africa call centre!!) the refund is outstanding and the voucher number I was given is invallid. I am utterly disappointed with the service from Fly Africa. I have also sent numerous emails to [Email Removed] which have gone unanswered. This is totally unfair and unacceptable.