The switch is the easy part, but they are pricey.
The hard part is control. As I see it there are these options:
1. Physical (The one you want to eliminate)
2. Phone
a) A separate line
b) The tennant's line. My alarm system answers with a tone when you call the hose two times in a row. This can be used for remote programming.
Anyway, options include voice prompts, 4 digit pass code, DTMF commands or a modem you actually call, but that would require a computer doing the actual calling or maybe something else.
Their phone? Your phone? They probably don't have a phone or just use cellphones.
Maybe a dial-up Internet provider still exists so you can send text messages. Maybe you could be a dial-up internet provider. This means that at your home have an extra line that provides PPP internet access. MAybe even an "On Demand thing". Call phone, enter 4 digit code. System uses voice prompts. Then it hangs up and connectes to the internet using PPP. Now, you interact with a Webpage or Text messages. System times out, so toll calls don't continue to be made.
Advantages. A low cost phone line at both ends. A wierd access arrangement, but it could work. Remember, the connection would be slow.
b) The Internet. Probably simple enough, but you at least need a dynamic DNS service at the location so you can reach it and maybe some wierdness that it try to ping somewhere and if it doesn't get a response it initiates Shutdown. Sub options, Their internet or your internet.
Lots of stuff exists to control and monitor over the Internet.
c) Cell phone Technology. Basically a Cellular modem or Cellular connection. Pay for a cell phone plan with TEXT messages or an Internet plan.
See, the utility has it covered. Zigbee to go house to house for a few hundred houses and a Cellular or other internet connection for all of that traffic.
You have to deal with "Tamper alarms" and possibly a waring system that your utility will turn off in 5 days unless the rent is paid.
I have no idea what the warning system might be. Flashing lights. Text messages. Emails. Automated phone calls, whatever.
These are some ideas and I don't have the foggiest of how to implement them except conceptually, it's possible.
I won't get into legality either. You still need power to run the system even when it's off.
AND...
If the utility is in your name, what prevents you from calling the power company and ask for a remote turn-off via the smart meter?
Don't forget to charge a re-connect fee.