I am unclear on your meaning (the original post) . You can clear a material mostly by right click and select all nodes, then select delete. Another  way is to load a simple material and then select the script (macro) to delte unused nodes.

 

Deleteing a material zone can be done in the group editor or by using a text editor in the obj file. the usemtl lines are the definitions. You need to assign those to another material though, just deleting them will be a mess.