There's a Red Power Button in the Top Right Hand Corner on the Digital Remote if you're using the App to control your Roku Smart Soundbar. Took me 2 months to figure it out. It won't turn off the TV, but it will turn off the Soundbar (but not the subwoofer either).
When you're done with the Roku Soundbar for the night, hit the power button in the App and it will turn off. The physical remote will only work for your TV if that's how you initially set up your device. You'll have to use the remote on the Roku App to turn the Soundbar back on.
The handheld remote will only turn your TV on when you hit the power button on it the next time you use your Soundbar. I thought maybe if I turned off the TV and then the Soundbar with the App that I could use the handheld remote to turn them both on, but only the TV came back on. Oh well!
At least I can give the speaker a rest and turn it off at night. Oh - my subwoofer auto connects the next time I power up the system when using this method. Hope this solves some issues for others.