How can i disable it? In my bios it's already turn off. But earlier ago. When electricity flactuated. After power loss/ My pc suddenly reboot. Im expecting to turn it off after flactuated but its not. My motherboard is msi g31m

Hello avelnatz32, welcome to PCHF.

The power must be completely cut for at least a second or two before the PC will switch off. If there is even a small amount of power to keep the PC alive, the lower power will be perceived as an onboard fault rather than a power cut, which would reboot the PC instead of shut it down.

I wouldn't normally suggest this, but as a test, when your PC is doing nothing, disconnect the power cable for a couple of seconds and reconnect. The BIOS is the only software method of shutting that PC down after a power cut.

EDIT: Marked as Answered.