Open up the .obj file in a text editor, find usemtl at the head of the f lines, rename this to usemtl  "your material". Delete all other instances of usemtl and "name".You will now have one material named "your material". Similarly, if you have multiple groups (g), you can use the same process to quickly create a single group encompassing the object. Never ran into any file corruption using this method, .obj files aren't very labyrinthine.