3D Voronoi Subtraction

I have created a 3d voronoi by creating a set of points in Rhino and then using the '3d Voronoi' tool that looks something like below:

What I am wanting to do is remove some of the polysurfaces indicated in lime and numbered 1-4 so that I end up with the base to create a structure like this:

http://www.grasshopper3d.com/photo/3d-voronoi-sculpture/prev?contex...

Any help would be most appreciated.

Michael