PIMS allows the user to make changes to table BLNMIX in a case. AU does not allow changes to which blend stocks can go to which blend. Since this could be construed as a flowsheet change, a suggested way to do this is to add MIN and MAX columns to the Blend Component tab for each blend. This would allow the user to set the MAX to 0.0 for cases or the base model when they didn't want this blend stock to be added to a blend.
Note that this MIN/MAX capability is not in PIMS by blend, only by blender.