If your player cheat then as GM do what you need, cheat.
He has roll something but you forgot the -30% malus for any reason, ennemy will begin to never miss him, and be strangely effective with their guns, make him use fate then burn some. At a point in the game he will turn to you and ask why?
Then you could answer an eye for an eye a fair game for a fair player.
If nothing succed kill the character and hire a new player. RPG is a hobby, a pleasure to have with friend a good time. You have no obligation to keep a fun destroying player, never ever.