From the info you supplied me, when watching your dvd there is already subtitles in it. If you can't disable it on your dvd that means that it's burned into the video and can't be disabled. However if the subtitle isn't burned into the video u can disable.
One way to erase subtitles taht's already burned into the dvd is to crop that part of the video out, however the bad part is you lose that part of your video.