Poser will build a corresponding MTL file for any OBJ it uses that does not have one. If you don't want Poser to build a MTL file when you import your OBJ make sure there is one already there. It's not a Poser fault that it builds a MTL file without asking you anything. It isn't supposed to ask you. MTL generation is automatic upon OBJ file creation in most software and Poser expects the file specification to be followed. If the corresponding MTL is missing Poser will generate one.