I don´t know about turning it off for certain assets or emitters, but wouldn´t it be easier, if you just turn refraction off for this smoke material? Hook up a parameter to the specular input for that smoke material, and set it to zero. Remember, in raytracing, the refraction is driven by the specular input, NOT by the actual refraction input. And the specular input has a default value bigger than zero, therefore it will come with a default refraction, if the input is left empty.
Or, if you want the smoke have refractions, you could use a mask/texture instead of a parameter, black = no refraction, white = full refraction.
Edit: Ah, have tested it, and the edges are still visible on a fully transparent plane with zero refractions :eek: