In Blender 2.75a, I'm making a render using Cycles and the Freestyle pass. My scene contains a mesh with a smoke modifier that I'm rendering through a volumetric shader.
The boundaries of this mesh (the box, in the example below) are meaningless to the extents of the render, therefore the mesh should neither have its edges drawn by freestyle nor cover the lines of the things behind or inside it.
I don't know if it's possible to tell freestyle to completely ignore the mesh, and how to do it.
This is what happens if I render a sample scene with cycles+freestyle+volumetric rendering and without further options (left) and with the box assigned to an excluded object group (right)
Neither of them is the expected result: I'd like to have the freestyle lines on the sphere but not on the box. Is it possible?
Add after TLousky answer
As you can see, even if I choose to render freestyle all the objects except the box using an "Inclusive" group, the "phantom" of the box still covers the edges of the sphere and partially of the cone and torus (exactly as in the example above where I used an Exclusive group containing only the cube instead)