There are many aspects of cg lightning and many ways to achieve better realism in renders. I think you'll get more help if you will ask more precise questions.
Im not sure what you are trying to show In "End of the world" scene but maybe it can help you:
To exclude objects from lighting by particular light the simplest way is to open the object properties window. There are two important tabs: Render and Lights where you can choose wheter the object casts shadows and reflects the light from particular light source. You can turn off some lightning effects like radiosity for any object in scene too. In the outdoor scene like you want to create, many things depends on how you want to simulate the sky in the background. For example, you can use the HDRI image and load it to "Image World" [Scene tab, then open Effects window] to generate backdrop for objects in scene and light the scene too - then the sky won't be affected by purple lights and will stay dark. You can build a big dome object above the scene and texture it with stars. Then, if you will make it 100% luminosity, 0% diffuse and will turn off the shadows the effect will be similar. If you're planning still image you can even add the sky after the rendering in postprocessing by using an alpha channel of rendered image. There are many ways - I'm sure you'll get more and better help if you'll give us more details abut what u want to make.
