Cell C - Cancelling of contract

Once again I am getting bad service from Cell C. I want to cancel my contract as my 24 month contract has expired and I would like to go pre-paid. I sent a letter requesting the cancellation to Cell C, and have been phoned twice by Cell C to verified my details for the cancellation. When given my verification details Cell C say that is not what they have on their system. I have been a customer with Cell C for plus minus 15 years and have previously verified my details when contacting Cell C without any problems, now it is wrong. I will not advise anybody to get a contract with Cell C, because they are quick to give you a contract but if you want to cancel it is a battle.