FUTILE CANCELLATION SERVICE

I cancelled my Telkom account on the 1st of Feb 2018, but continued to be invoiced. I then notified Telkom yet again of this cancellation on the 19th of March, but once again was billed. I have now received a letter of demand, for a service I cancelled in February. As usual it appears that someone's not doing their job and now I'm being penalised for it. I am livid that because someone has failed to do their job I must now be liable.