If you use Group Policy or Registry settings to block the changing of the desktop theme, you might note that this doesn’t seem to block the hotkey combo for setting High Contrast Mode. HOWEVER, it does block using the key combo to change things back.
Found a registry setting which disabled the hotkey combo, but still allows it to be set manually if allow it.
Set a REG_SZ named “Flags” with a value of 122 (126 appears to be default).
Once the user logs off and logs in again, the key combo won’t work anymore!