You would have to disable handles from the NGUI menu to do this.
Keep in mind NGUI was designed in a way that you wouldn't have to do this. By default, colliders auto-adjust themselves to match the widget's bounds.
If you need an easily resizable collider that doesn't have anything visible associated it, create an invisible widget (ALT+SHIFT+W) and give it a box collider (ALT+SHIFT+C).