Seems the IsReadOnly option does not work as expected on the ComboBox. After a little searching I found that the IsReadOnly option affect the ablity of the user to edit the text in the ComboBox but does not affect the user's ablity to change the selection which is what I wanted.
To work around this first you need to set the IsHitTestVisible to false, setting this option to false means that none of the mouse events are sent to the control so the user can not select a new option.
This does not stop the user for tabing to the control and selecting a new option with the keypad. So to stop the user from using the keypad I set the IsTabStop option to false also. This way this control is not on the tab list which means it can't be selected.
So by setting both option the user can not longer change the option in the ComboBox.
first test
second test
third test