Poser regenerates materials slots from any materials entries it finds in the geometries file.  Even if you delete all the materials definitions, the next time you open the figure in Poser it will put the entries back in (even if they no longer have the correct material definitions; for instance purple skin and blue eyebrows).

If you have saved, and re-started Poser, though, you should get only materials for the geometry of the figure you've loaded.