Unfortunately it's not currently possible to do, as DisplayFusion has to generate one giant stitched together image that has the images for all monitors because Windows 7 and below only support setting one image as the wallpaper. For Windows 8 and up, we're hoping to add the ability to just use the Windows 8 APIs which do allow for an actual separate image file for each monitor.

I'm not sure if that will allow us to exclude a monitor altogether, but we'll definitely let you know if it does

Thanks!