There are a few ways, but here are two of the most common.
Open up a schematic view and turn on the material display option. This will show all the materials, select the ones that you don't want and delete them.
or
I've got a free plugin on my web page www.EdHarriss.com (under plugins, Softimage3D) that will delete all the materials and textures on the selected objects. It's called Blackout. After you install it, select the object, then chose Model>Effect>Blackout.
Hope this helps.
Ed Harriss, http://www.EdHarriss.com
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