Disable Screen Auto Rotation on Unity3D

Disable screen auto-rotation in a game in Unity. 

Option 1

Create a script that disables auto rotate to portrait and sets the orientation to landscape.

using UnityEngine;

public class CameraScript : MonoBehaviour {

    void Start() {
        DisableAutoRotation ();
    }

    public void DisableAutoRotation() {
        Screen.autorotateToPortrait = false;
        Screen.autorotateToPortraitUpsideDown = false;
        Screen.orientation = ScreenOrientation.Landscape;
    }
}

Create a game object to attach the script to. This can be any game object. You could also attach the script to the main camera in the scene either. When the game is run on hand held devices, the autorotation will be disabled.

Option 2

Another solution to this problem is to change this setting when building the project. 
Go to 
File -> Build Settings...

Click on Player Settings...

Click on Resolution and Presentations panel in the inspector. This will then expand and present more options.

In the Resolution and Presentation panel there are options for configuring the orientation.

First select the default Orientation. This is the orientation that the game will start in. 

You can configure what orientations are allowed when the game is being run. To do this, check or uncheck the required boxes for "Allowed Orientations for Auto Rotation".