Holding the power button down for five seconds or so should trigger a shutdown. When that happens it will display the powered off screen. If the device has hung, holding it down for longer than that, will force a power off. I'm not sure how long is needed, but, I always say 20 seconds in the factory reset instructions. When you do this, the screen doesn't change.
And the manual I found on the Kobo support site, has instructions for turning the device off on page 59. The manual is at
https://kbdownload1-a.akamaihd.net/m...erGuide-EN.pdf.