IMEI Blacklisting - FNB Connect

I lost my cellphone 2 weeks back and I requested IMEI to be blocked I was told that they cant Blacklist IMEI number I gave to them reason being that MTN already blocked another IMEI number as the cellphone was a Dual-Sim.

Now my insurance is taking me back to FNB to blacklist that Line. Please FNB kindly assist me ASAP otherwise you will have to replace my Cellphone if you cant Blacklist that line.