Blacklist cellphone

I had an armed ******* at home and my spare cell phone was ******. I contacted the Vodacom call centre to black list the phone as the last number used from that the phone was my Vodacom number and they are not able to assist. I have the IMEI number but they advised they can not black list the phone using the IMEI number. Please advise how do I go about blacklisting the phone.