1. Press NOW button.
2. Select the "Wheel" icon for Settings from the pull down menu.
3. Select TV Settings from the pull down menu.
4. Go to Video & Audio Settings on the left.
5. Go to Dolby Digital Audio on the left and choose off for HDMI Digital Audio Output.
Then you can check if the Dolby Digital output from the optical audio output has also been switched off.
Since I do not use the STB's optical output, I ccould not check if the above works or not.