Bill settled however still getting invoices

I have paid up my outstanding amount for my modem contract last year in September. However Vodacom then debited my account again in October after the settlement. I've been in contact with vodacom to cancel my contract since the two years ended in August and asked for a refund since I paid twice. It's been two months now that I've been receiving invoices for the same number of the modem in-spite of settling the account and asking for vodacom to cancel the contract because I do not want to upgrade. Now they tell me I've upgraded this year in January however that modem hasn't been in use since last year and I'm not the one who requested the upgrade. What ****es me off is that this is not the first incident I've had with vodacom regarding upgrades and cancellations. I'm now even thinking of taking legal action against vodacom because they are tarnishing my name.