Billed for a cancelled contract

I app**** for Data only sim from MTN, i was looking for something that i can use when i am not at home because at home i am using fibre. They sent me a sim towards end of March. The sim card did not work (had no internet connection) and i have never used it not even once

I tried to call customer care but there was no response for 2 weeks. I sent e mails as well but they ignored my e mails as well. I tried the customer care again and finally got hold of them. When i finally did i managed to cancel that contract, i even received a confirmation sms from them to show that contract has been cancelled which was on the 1st of April. Then during month of April they started sending me statement that i owe R128. How because the contract was cancelled and i have never used that sim because there was no network connection?. I sent e mails and attached proof that contract was cancelled but they choose to ignore me. Even today i am still waiting and i am receiving annoying sms's that i have to pay. Why would i want to pay for something that i have never used. Looks like no one is willing to resolve my issue