The 'material zones' are part of the geometry, and the geometry is usually stored in an obj file, thought it can also be stored in a 'library' file (eg cr2, pp2, hr2) as a 'geomCustom' section. The sections of the geometry that define a 'material zone' start with the word 'usemtl', and this is followed by the name of the material, next comes a line that names the 'group' (g) that the zone belongs to, and that is followed by a line or lines that defines which facets comprise the material zone.

The settings for a material (as opposed to the material zone) are stored in the 'library' file as 'material' sections. When you delete a 'material' section from a library file, if the 'usemtl' section still exists in the geometry, Poser will create a new 'material' section for that zone the next time the library file is loaded.

To delete a 'material zone' you need to delete the 'usemtl' section for that material from the geometry (obj, or geomCustom). You do that in a text editor or cr2 editor. For the sake of tidiness you should also delete the 'material' section from the library file.

The above is given as information for anyone who wants to delete a material. I am not suggesting that you should delete materials in you particular circumstance. Most of the time, the easiest way to achieve whatever aim you have in relation to materials, is change the material settings.

Poser will always load a material named 'Preview', even if no material zones are defined in the obj.