This is a very recent problem. When I turn off my TV, it just turns back on again. I have traced the problem to my Roku Express. If I physically disconnect it from the HDMI input, the TV stays off when I turn it off. I read online that I should make sure the Roku is on the home screen when I turn off the TV. I tried that, but it doesn't work. The TV goes off and then comes right back on again. The only way to make it stay off (with Roku connected) is to hold down to physical power button on the TV for several seconds. I've looked in Roku settings but I don't see any obvious answer there.
Try unchecking "1-touch play" under Settings->System->Control other devices.
I tried that, but the TV still comes back on after I turn it off. To clarify, my method of disconnecting the Roku from the HDMI is actually to switch the active HDMI port to a different device (Chromecast) using an external HDMI switch. With the Chromecast screen displayed I can turn off the TV and it stays off.