Maybe download app "Rotation Locker" (by madmack) from Google Play Store.
It enables you to (generally) enforce either Portrait or Landscape mode or "Auto".
I myself use it (for the opposite purpose as you) on my Google (Asus) Nexus 7 tablet (which natively only shows portrait mode).
Another (and maybe a better) option for you is to use BSPlayer's own features:
While playing a movie, tap screen and then tap menu (at top-right in the bar).
Then select "Preferences" followed by "General Preferences"
Scroll down and tap on "Screen orientation"
Select "Portrait" (or "Portrait (sensor)")