Unable to cancel unwanted service

In January the area's ADSL lines were upgraded and Telkom contacted me to say agent will come to my home. This agent basically bulldozed me into signing on to upgrade to most expensive ADSL line and service. I was very hesitant and said that I need a month to cancel my existing service so not to submit any forms. I contacted Johannes Tshililo again that night to tell him not to go ahead with the contract as I didn't want a contract-based service and it was much more expensive than my existing service. By the following week I received an SMS from Telkom informing me that the service has been added. I contacted Johannes numerous times to cancel the service and was assured his manager would do so and send me a confirmation letter. Even after enquiring again a number of times, this never happened. I cancelled the service at a Telkom store, only to receive an invoice for 2 months for this service for Feb and March. Trying to get hold of someone at the Telkom call centre is futile. The phones simply aren't answered and if someone answer them, you are tranferred to another centre who cuts you off without answering. I've struggled unsuccessfully for a month to cancel this unwanted service.