How to clear GCS Transformation in ArcMap data frame?

Hi Everybody:

I am trying to display all WGS84 (feature layers) and NAD83 (image layers) data layers in ArcMap at the same time. Here is what I did:

1. all the layers are added to the same data frame of TOC
2. I looked at the Coordinate System-->Transformation information of the data frame property, and found that it converts NAD83 to WGS84 using "none" (see attached picture noneTransformation.PNG)
3. I checked the displayed features and the aero map image, found that they are matching each other (which means the display is most likely correct)
4. I open the data frame property Coordinate System-->Transformation information again, change the conversion to be using some transformation from the drop down list (see attached pictue someTransformation.PNG).
5. I checked the displayed features and aero map image in the map again, found that they have some offset, which means the display is definitely not correct.
6. I open the Transformation information and attempt to change the transformation to "none" again. However, I found that it does not allow me to do that. I reopen after the change and found it is still the previous transformation.

Why I cannot change it back to none?

Thank you!

Attached Thumbnails