Ongoing Charges for Contract that should have been cancelled.

Data contract taken and returned within 7days as there was no network coverage, even though MTN Kyalami checked and confirmed coverage before the contract was taken. The call centre retention insisted I return the moderm and sim to the store and on returning it the mtn ladies and the store confirmed the contract will be cancelled and that I could leave. Since then I do not have the moderm or sim and have not used even 1MB of the contract and returned it all within 7 days but I am being charged and retention is not canceling the contact. my Ticket no. is :*** and I have been holding on the line for retention more than an hour each time I call, but no assistance. The store admits they have the device and sim but that it wasn't cancelled from their side, further there is nothing further they can do but keep escalating the matter to retentions.