MTN not cancelling my contract as requested

I have taken out an wifi contract, and I have tried to cancel it on 30 October, I am still receiving billing from them. I have given back the wifi router after an hassle, I had to go to the store 3 times jus to give them back their router. I have phoned them, I have mailed them and I a confirmed with the store. I have done this all within the 10 day cancellation period, I have never used the wifi router.