Telkom Refuses to Close Contract

I've paid up the contract in November, Telkom sent me another bill for December and I paid it too, I requested closure of the account and migrated to prepaid, but Telkom still sends me monthly bills claiming that I owe them. I've requested a letter when the account was closed in December but none received to this date. The number is on prepaid but I get bills for contract, they tell me is for December and I sent them proof that it was paid and sim changed to prepaid but they refuse to close it. This is the most useless network with *********** staff one had to deal with. This incompetency is now affecting my credit profile.