Using Oxygen, open the model you want to untexture.
Select the faces you want, or Ctrl-A for all.
Then go, Faces -> Face Properties, and remove the name of the pac or paa file from the bottom right box.
If there is more than one texture in use you will just have 3 dots (...) in the box at bottom right. Just delete the 3 dots and click apply and it will remove the textures from selected faces.