Debit on closed contract

I had a Sim contract R99 for 1 gig data with Telkom for a year and when the agent from telkom phoned me I canceled the contract. Telkom continued to debit me the R99 and everytime I reversed it they charged me double, this when on for 2 years until I changed my bank account details. This is poor service from a big service provider and very unprofessional.