non-terminating mtn contract

I emailed mtn in august about termination of my contract. They responded it will terminate end October. Now in November i get an invoice. They claim they've moved me to pre-paid, which is just nonsense. Pre-paid means you pay before use, so how do they invoice for a pre-paid? It appears these guys have come up with a formula to fleece money from clients after their contracts expire. I'm not sure what the NCC and the Ombuds are doing about this malpractice. I'm also not sure why mtn management is continuing to profit from this ******ion. When a contract is done, it's done. Please stop these shenanigans!!!