How do you lock the first-person CharacterController to only walk along the x-axis?

I am making a 2.5D sidescroller and want to know if there is a way to take the built-in CharacterController and either: 1. Lock the movement to be only on the x-axis and y-axis(so that the character only moves left and right and may jump) or 2. Remove the keys W and S and UP and DOWN so that there can be no movement on the z-axis. I have read other "fixes" where people place invisible walls so that the character is restricted from moving along the z-axis, but surely there is a real fix to the problem. THANKS!