Did you try a simple subclass approach and trap the CBN_SELCHANGE notification by overriding WindowProc, which is virtual? You can decide to ignore the notification or not.

One approach would be to subclass the windows control GetClassInfo and RegisterClass. Then in your own window proc (or message map if using MFC), you can ignore the sel changed message using a method to turn it off and on.

Once registered you can drop it on a dialog using the custom control in the toolbox or you can use SubClassDlgItem and to swap out the control using code when the dialog is created.

Either way, you have to be careful about what to do if the CBN_DROPDOWN message arrives as users can arrow down and enter or just move down and click an item.