Contract Cancelation

We tried to cancel our 2 year contract with Telkom, which is on my mom's name. We were told that if we cancel the contract, they will terminate the sim card that we have been using. Now we haven't received help in any form since 2 months ago. They not providing any further assistance instead they keep on selling us their products. What should we do to cancel a simple contract? And how does this company terminate a sim card that we have been paying for.