At the moment the only way to restart Portal (ie. power down) is to change the language type in settings or unplug the physical device itself.
I've had two Portals now where Alexa becomes unresponsive and restarting seems to be the only fix.
(I'm guessing this bug is related to having multiple portal users or multiple Alexas in the house and I'm hoping it will be fixed in a later update.)

that said having an option in settings or "hey portal" command to reboot Portal would be a benefit, even if its just to clear Ram and keep Portal running nice and smoothly.