Conversations with a pair of programmers who have written multi-display utilities  are indicating that there probably isn’t currently a function call to power down one monitor in a Windows extended desktop multi-display setup.
So I guess that obliges me to bring this issue to the attention of Microsoft and Samsung, as a future feature recommendation... all in good time.
 A pair of programmers who have written multi-display utilities:
“I don't know about any way to programmingly turn off a single monitor in multi-monitor systems.”
~ Nir Sofer
– NirCmd and MultiMonitorTool, at NirSoft
“Wizmo, and I guess NirCmd as well, use the Windows Power Management API to power-down the screens. What I don't know is if this API allows you to specify a particular monitor or if always applies to all monitors. If I have time next weekend, I'll take a closer look at this API.”
~ Gerald Evans
– Dual Monitor Tools, at SourceForge