When SheetCam loads a dxf, it ignores the Z depth. As far as it is concerned the lines on the surface are perfectly valid. If you put them on a different layer you can hide them in SheetCam. If you go to view->layer tool, you can hide individual layers. In SheetCam you can move lines on to different layers by clicking on the 'edit contour properties' button (a C with an arrow next to it). You can then click on lines to select them (use <CTRL> click to select multiple items). Right click and select 'move to layer'.
You need to be a bit careful loading 3D dxf files. As SheetCam ignores the depth you may find that you end up with two or more overlapping outlines. This can cause all sorts of problems. It is best to convert the dxf to 2D before importing it into SheetCam. I am sure there are some Rhino gurus in this forum who can tell you how to do this easily.