Just to clarify does this happen when the TV was in standby and the power is cut off or only when the TV was on and the power cut off?
To better assist are you able to advise why your switching the power off at the mains, is this due to concerns over electrical fire hazard, concerns over wasted energy while on standyby, or something all together different?
Unfortunately this is a case of poor design by Samsung.
The control board remembers the last mode of operation, eg Standby, which channel etc, and when power is interrupted the TV isnt smart enough to just default back to standby.
This isnt just a problem with your model I am aware of the same issue on the 8000 series too, the standard steps Samsung will tell you are to check the firmware is up to date, but confident this wont resolve the issue.
So unfortunately there isnt a setting which will enable you to stop the TV coming back on when the power is restored.
Good morning sorry for the delay in responding, do you still require assistance?