Well guys the name changes back automatically after the round finishes as i remember (because of that addon or whatever it is) , all you should've done is to change hes nick with amx_nick and kick him afterwards , that way when he come back he would've stay with the nick you put on him .
Sorry if i'm wrong .
P.S. Why didn't you made a pic of the message in which you warn him to change it ?
Peace .