In reply to wikkit, "pin 0 does not do anything". AFter lot of frustration and trial and error, I found out that pin 0 does actually work, but only if the tiny lily is powered externally from a battery, and not connected to to the USB of my computer.
Even trying Serial.end() does not disable serial mode if connected to the USB cable to computer.
Even trying Serial.end() does not disable serial mode if connected to the USB cable to computer.