You can't refer to what's on the shelves. Why do you want to force it to null - just because you don't want people using it - is it part of a heirarchy?
The reason for not displaying it is that the data will not reflect the correct numbers. There could, for example, be two sub sub segments underneath a sub segment. To make the data display correctly (in sub segment view) I will put the number which should be shown at the sub segment beneath one of the sub sub segments and leave the other blank. We do not want people drilling to the sub sub segment and getting incorrect information. It can be part of a heirarchy if this will allow for the nullification.