Closing of account

I was in Botswana last year, and I had to leave and I am back south Africa. And I have sent them countless E mails and I sent them my passport number my account number and I sent them a letter saying I would like to cancel my account and I have signed with my signature on it and with the date on it as they requested with my account number on it. And they have not come back to me. So please what must I do. Regards Terry Charl Marks