Those are 'hardcoded subtitles' you're describing SJ.
And you're right, they can't be removed.
'Forced subtitles' are the ones that can't be turned off, unless of course you edit the appropriate 'Prohibited User Operations' (PUOs).
The 'Subpicture stream change' PUO being the relevant one in this case.
As far as I recall DVDFab removes all PUOs from both the IFOs and VOBs by default, except for the 'angle change' PUO (since removal of this can cause minor problems on certain standalone DVD players).
Earlier versions of DVDFab actually removed the 'angle change' PUO as well but Fengtao fixed this after I'd highlighted the difficulties.