I agree with the other posters- teach the kid or other offender, or buy one
of those clear lock-boxes with the vents, to install around the thermostat.
Any wall-mount lock-box could be adapted, just drill enough holes for
venting, but the clear ones are slightly less ugly. Might have to go to a
real supply house, not the big-box, to find one. Or maybe if there is a
cover over the buttons, just devise some sort of lockout or seal for that.
If you are handy, you could even build a recessed cabinet into the stud
space, mount the 'stat in that, and put a locking glass door on the front,
like an old-time office fire alarm. Again, be sure to vent it, so the thing
gets a true reading of air temp.

aem sends...

Reminds me of a story I heard where the tenants kept an ice pack on the
locked thermostat in the winter time to get the house to heat up more.
I suppose you could do the reverse with a small space heater blowing on
the thermostat in the summer to get the A/C to go colder.

Ken