I used to have that problem and it was an annoyance.
It would occur whenever I Re-Authored a movie and at first I used to use the option on the standalone DVD player to turn the subtitles off.
If you are re-Authoring as I suspect, right-click on the main movie title that you select on the left (DVD Structure) and select Set default streams.... In the Subpicture box, select Hide subpicture.
Now you subpicture (subtitle) will not be displayed by default.