How do I stop my camera flying?

I'm experimenting making a fps camera for a possible game and I'm having a bit of trouble. I've managed to make separately make the camera move along the x & z axis and use the mouse to rotate the camera but when I point the camera upwards while pushing the up arrow key, the camera moves upwards as if I'm making it fly, which is not what I want. I want the camera to stay fixed on its y axis so even when I point upwards, it stays "grounded". Is there a solution

Thanks Here's the code I used, maybe there's a mistake

void Update() { var x = Input.GetAxis("Horizontal") Time.deltaTime 3.0f; var z = Input.GetAxis("Vertical") Time.deltaTime 3.0f;

     transform.Translate (x, 0, z); 
 }



 Vector2 _mouseAbsolute;
 Vector2 _smoothMouse;

 public Vector2 clampInDegrees = new Vector2(360, 180);
 public bool lockCursor;
 public Vector2 sensitivity = new Vector2(2, 2);
 public Vector2 smoothing = new Vector2(3, 3);
 public Vector2 targetDirection;
 public Vector2 targetCharacterDirection;

 // Assign this if there's a parent object controlling motion, such as a Character Controller.
 // Yaw rotation will affect this object instead of the camera if set.
 public GameObject characterBody;

 void Start()
 {
     // Set target direction to the camera's initial orientation.
     targetDirection = transform.localRotation.eulerAngles;

     // Set target direction for the character body to its inital state.
     if (characterBody) targetCharacterDirection = characterBody.transform.localRotation.eulerAngles;
 }

 void Update()
 {
     // Ensure the cursor is always locked when set
     Screen.lockCursor = lockCursor;

     // Allow the script to clamp based on a desired target value.
     var targetOrientation = Quaternion.Euler(targetDirection);
     var targetCharacterOrientation = Quaternion.Euler(targetCharacterDirection);

     // Get raw mouse input for a cleaner reading on more sensitive mice.
     var mouseDelta = new Vector2(Input.GetAxisRaw("Mouse X"), Input.GetAxisRaw("Mouse Y"));

     // Scale input against the sensitivity setting and multiply that against the smoothing value.
     mouseDelta = Vector2.Scale(mouseDelta, new Vector2(sensitivity.x * smoothing.x, sensitivity.y * smoothing.y));

     // Interpolate mouse movement over time to apply smoothing delta.
     _smoothMouse.x = Mathf.Lerp(_smoothMouse.x, mouseDelta.x, 1f / smoothing.x);
     _smoothMouse.y = Mathf.Lerp(_smoothMouse.y, mouseDelta.y, 1f / smoothing.y);

     // Find the absolute mouse movement value from point zero.
     _mouseAbsolute += _smoothMouse;

     // Clamp and apply the local x value first, so as not to be affected by world transforms.
     if (clampInDegrees.x < 360)
         _mouseAbsolute.x = Mathf.Clamp(_mouseAbsolute.x, -clampInDegrees.x * 0.5f, clampInDegrees.x * 0.5f);

     var xRotation = Quaternion.AngleAxis(-_mouseAbsolute.y, targetOrientation * Vector3.right);
     transform.localRotation = xRotation;

     // Then clamp and apply the global y value.
     if (clampInDegrees.y < 360)
         _mouseAbsolute.y = Mathf.Clamp(_mouseAbsolute.y, -clampInDegrees.y * 0.5f, clampInDegrees.y * 0.5f);

     transform.localRotation *= targetOrientation;

     // If there's a character body that acts as a parent to the camera
     if (characterBody)
     {
         var yRotation = Quaternion.AngleAxis(_mouseAbsolute.x, characterBody.transform.up);
         characterBody.transform.localRotation = yRotation;
         characterBody.transform.localRotation *= targetCharacterOrientation;
     }
     else
     {
         var yRotation = Quaternion.AngleAxis(_mouseAbsolute.x, transform.InverseTransformDirection(Vector3.up));
         transform.localRotation *= yRotation;
     }
 }