blacklisted phone still active

I contacted the service centre on 173 to blacklist a ZTE *** (+***) ****** by **** and obtained a reference number from NTSAKO. I was promised that the phone will be off in 24 hours. The phone is still active and even after contacting the service centre over 22 times, with the call being dropped in my ear 21 times. Every time I request to speak to a supervisor, I was given the run around and call was dropped. Everytime I am told that MTN can't block a prepaid card because they don't have a system to do it! Whilst writing this report I have been holding for over 20 minutes on second attempt listening to the IVR \in the order which it was received"repeating over and over again. I have contacted the number and even though I was told that the phone will be blacklisted"