Demanding payment for settled and closed accounts

I receive calls from this people demanding payment from me saying they are acting on behalf of CellC. I usually tell them that I don't have a contract with CellC and they'll ask if I didn't have a contract with CellC and my response will be "I once had", and they will say you didn't cancel the contract. I told them that I cancelled the contract two months before the end of the contract and I do have a confirmation SMS of the cancellation. I received an email yesterday that tells me that they're going to blacklist me if I don't pay within seven days. I rep**** the email by sending settlement letter for one account which I paid to CellC debt collectors two years back and the other account's last statement of zero balance and it's cancellation SMS. Today I called CellC about these people after answering their call and the CellC consultant told me to block them. They did the same thing last year same month of August using another name of CSS Solutions and started again this August. I told them that I sent them documents last year and they said I must resend. I did yesterday, today they call demanding payment without saying anything about the email that I sent yesterday.