Bad Service from sales consultant.

I'm currently a customer of Cell C for some time now - Awesome service - until now - Cell C phone me offering me a better Data deal - I have accepted this offer and then all of a sudden I receive two data packages and being billed for them... I informed the sales lady that I need to cancel my smaller data package with them - so they put me through to the cancellation department to do the cancellation - today I have phoned again to confirm cancellation to find out that it was never done!!! Now I will be billed for stuff that I don't need and or use. How long will this last for them to actually cancel -