Mtn undermining me

I'm Jacob phago ID no:***.Two years back I ordered router and sim from mtn.They delivered only the sim and I called them to cancel the contract.They have agreed to do so but it's the third year now they appear on credit record as I owe them.Last year I called they said it's cancelled n they will sent me cancellation letter till today and the problem still exist.Mtn is tarnishing my credit record please help