Hm, that's probably the point: there is no threat (to the utility provider) in that model. Worst thing you could do is make someone's appliances run with a more expensive rate; hardly a likely event or one worth preventing.
(As for power cutting, that seems unlikely - the system is in place to provide larger power consumption when base production exceeds the load. Around here, the basic low/high rate info is provided by https://en.wikipedia.org/wiki/Load_management#Ripple_control , which needs a not-really-smart power meter, but runs on top of the power line, obviating the need for a side channel)
The smart metres that are being rolled out at the moment have a remote kill switch, although it couldn't really be used to implement rolling blackouts as you have to physically hit a button to reactivate it.
That's a DoS waiting to happen, IMNSHO. If it's easy to transmit on the frequency locally at low power and there's no auth, a drive around the neighborhood could create a blackout at anyone's demand.
At least in the UK, smart meters don't use radio-based time switching as they have their own clocks which are synchronized with their remote system. All commands to these smart meters (including the ability to remotely disconnect them) are cryptographically protected to ensure integrity and authenticity, and also have anti-replay protections.
Disclaimer: I've worked for a smart metering manufacturer.
(As for power cutting, that seems unlikely - the system is in place to provide larger power consumption when base production exceeds the load. Around here, the basic low/high rate info is provided by https://en.wikipedia.org/wiki/Load_management#Ripple_control , which needs a not-really-smart power meter, but runs on top of the power line, obviating the need for a side channel)