"Making it impossible to block a ****** sim card"

Why do we have to RICA sim card if it’s such a mission to stop a ****** sim card. I bought a cell c phone for my 15 years old nephew. The phone was ****** and the person who found the phone is now using the sim card .We became aware when we saw the what’s app application active and people complaining for swear messages they receive from the number . I called in cell c head office twice for assistance , so since I couldn’t answer the 5 questions about the phone activities I was advised to go to the branch with my ID as they are the only people who can assist in this situation. I went to The mall of the south branch with my ID were I was advised I need to call the head office for assistance. I explained to them that I had already done that twice and advised to come to the branch. The consultant at the branch further told me she will ask the same questions even though they had my ID no. on record , the sad thing is even after explaining my situation ,she can’t stop sim cards and the is nothing she can do I have to answer those questions. I even asked if the is any ******** thing that can happen they can trace the ID back to me she still agrees but she can’t assist. I RICA'd the phone what will it take for me to block this sim, the phone got lost on the 7th of August already and the perpetrator is still busy with this sim .I would really appreciate your assistance.