Turn off phone by holding the power button?
There were those Secret Codes...


  *#*#7594#*#*: This code can be used to change the "End Call / Power" button action in your phone. Be default, if you long press the button, it shows a screen asking you to select any option from Silent mode, Airplane mode and Power off. You can change this action using this code. You can enable direct power off on this button so you don't need to waste your time in selecting the option.


Hm, guess that's what you were lokking for, right? Wanna more? OK, Here's the source, Luke: Secret Codes of Android Phones. Not sure if it works on all phones, though...