Trying to blacklist a ****** phone

I have now tried 6 times to have a ****** cellphone blacklisted but to no avail. I have phoned the contact centre 3 times as well as visiting 2 different branches 3 times as well. The cellphone was used by my grandson and I have no recollection when was the last time he recharged the cellphone or the amount he recharged it with. Although in my name they still will not blacklist it if I cannot answer these questions. I have supp**** them with the IMEI number still not happy. I really do not know what else to do as they are extremely unhelpful and really could not care. I would not advise anyone to do business with them as the customer service is non existent.