Refusing to cancel a contract and trying to collect

I cancelled a contract with Telkom about a year ago and since then i have been trying to make sure that they cancel my contract on their systems. I was called by their retentions office for upgrades but kept telling them i have no contract with them. Today i still receive messages that i owe Telkom for a service they signed me up for regardless of my countless efforts to cancel. They have handed over the account to debt collectors even after i have sent and resent the cancellation form to them over 5 times, i am sure. I am so unhappy about this because it messes up my credit record.