I want an object to not be visible at all in a render. I am trying to contain some particles in a simulation and I need them to not get out of the enclosure so I want a transparent object there (when I render) to prevent them from escaping.
The following material setup should give you a completely Transparent material that cannot be affected by light etc.
You can add a material and set the specularity and alpha to 0 and disable shadows but the easiest way is to not render the object at all. Open the Outliner and untick the camera beside the object.