Edit your PCF.

On the Hard Button Properties tab of the System Page, you will find that the 'Cursor Nav' button has been programmed as Special Action 'Cursor Navigation'. If you simply clear this Special Action, the only way a user can enable is via the Setup Menus.

If you'd like to disable ability to get into setup, you can hit 'Tab' while editing items on the system page until you come to the button with a Special Action 'Tools' assigned. If you remove this special Action, user won't be able to enter setup to change the setting.

I advise against removing the 'Tools' Special Action as following a firmware update, you will need to get into Setup to turn off the CursorNav.