Lock an axis from rotating

I have a CharacterController that is rotating to always face where the camera is pointing but I want to lock an axis so that it wont lean back and forth when you look up and down.

void Update() {
    CharacterController controller = GetComponent<CharacterController>();
    Camera cam = GameObject.Find("CharCam").GetComponent<Camera>();
        if (controller.isGrounded) {
            currentSpeedX = Input.GetAxis("Horizontal");
            currentSpeedZ = Input.GetAxis("Vertical");
            moveDirection = new Vector3(currentSpeedX, 0f, currentSpeedZ);
            moveDirection = cam.transform.TransformDirection(moveDirection);
    	    transform.rotation = cam.transform.rotation;
     	    moveDirection *= speed;
        	moveDirection.y -= gravity * Time.deltaTime;
        	controller.Move(moveDirection * Time.deltaTime); 

I would just set the rotation values for the other two axes to be zero. Right before your transform.rotation line I would say

Vector3 rotVals = new Vector3(0, cam.transform.rotation.y, 0);

Then set transform.rotation to rotVals. Note that this assumes your controller works like the default FPS controller, which is that the camera itself never rotates left/right, only up/down. Left/right rotation is handled by actually rotating the entire controller (instead of just the camera). If you are actually rotating the camera left/right as well, this obviously won’t work and I would recommend using an entirely Quaternion-based rotation system instead.