I've added a subtitle stream to a film, but the subtitles appear in the simulator whether or not I've clicked the button that should enable the subtitles to appear. That button has the main film as a target, and for "subtitle" in the advanced tab I have "subtitle stream 1" and the "view" box is checked. For the main "start" button, the target is the same but the "subtitle" field is not set, and the "view" box is not checked. But when I view the film in simulator, hitting "start" gives me the subtitles. I've tried setting the subtitle field to an empty subtitle stream, tried not checking the "view" box for the subtitle stream. What's even weirder is that I even tried setting the button for the subtitling to "not set", but the subtitles still appear.
When you don't want subtitles to appear, you can choose a subtitle stream [one which will exist on the disc; see next paragraph] and uncheck the View box.
But if you want to be sure the DVD player won't show subtitles against your wishes, create an "empty" subtitle track to point your disc programming to. Right-click in an empty subtitle track and select Add Subtitle, or Add Subtitle at Playhead. You now have one blank subtitle in that track, and that stream becomes your "empty" subtitle selection. If you don't put a blank subtitle in the stream, that subtitle stream is not muxed on Build. (Otherwise, we'd always have 32 choices of subtitles—most of them blank—on all DVDs produced by DVDSP!)
It's also a good idea to make the "empty" subtitle stream S1, as DVD players have been known to default to ST Stream 1 when in doubt. And if that stream is blank, then you don't see subtitles. I've never had problems on my discs when I put actual subtitles in S1 (and blank in S3), but others have told me it's better practice to make the empty stream S1 instead of S2 or 3.