Contract ended still sending invoices

I had a contract with Vodacom it came to an end, written confirmation was handed to Vodacom at the end of November, they even switched my account over to a pre-paid. Vodacom confirmed that the last payment was end of November, final payment was paid and now they still sending accounts. Vodacom your service is utterly poor, your lack of communication is poor I can't understand how you're actually still in business.