the Compositing tag is what you are looking for. In the example you would attach one to the lightbox and uncheck ‘Seen By Camera’ – if you don’t want the reflection of the light/shadow box on the floor but still want the light on the floor around the red objects you would need to add the floor/plane/whatever you are using to the Exclusion tab of the Compositing tag and make sure the reflection icon is dark (the icons are – Transparency, Refraction, Reflection, Hierarchy) when the icon is dark it’s enabled so reflections fron the shadowbox off the floor would be ignored