blacklist threat

I receive calls and text messages from NDS telling me that I have to pay my cell c account. I explained to them that cell c cancelled that debt because they debited my account on the wrong date therefore I'm not liable for charges. The calls and smses don't stop. They are asking me for proof of the debt clearance from cell c , I told them that I don't have it, nds did not offer me any service they have to ask cell c for whatever documents they need. They are now threatening to blacklist me