Blaclisting of Cellphone

I have been calling MTN for the past 2 days trying to blacklist my ****** cellphone which was using a Prepaid sim card. I dialled customer care 173 and was advised to dial 135 then 135 advises I need to dial 136 for Prepaid and vice versa. I am really dissappointedwith the service, yet one has been such a loyal client and you keep debiting for insurance. I guess you like taking the money and not serve the clients, sies!