Are the thermostats wired? (My Sanyo has the thermostat in the remote control).
If they are wired, then you may be able to solve part of the problem by installing a second (hidden)
programmable thermostat in series with a wall thermostat. It could be set to limit the low end temperature.
Or, you could install some power monitors,
(This one will display actual $ or Pounds, in real-time).
And tell the renters keep track of their energy use, so they can pay you
for any excessive use. Allow them
x number of kWh a day (on average), for 'free'..