Lost phone and Sim causing my bill to be high

I have a contract with Mtn, it happened that i lost the phone and sim card. I couldn't blacklist the phone because i could give the IMEI number, so in August 2022, I greylisted the sim card. However I have been receiving a high bill from the Sim card that is greylisted. Every month i have to check that the number is still greylisted, which is always not case. Which means i have to pay high bill for someone to use the sim card, which i have reported missing. Please assist since i have been going back and forth with customers services with no assistance, as they will say its blocked, then the next month i am told that the high bill is coming from that same sim card.