tell her once again not to use the aircon without your permission and if the bill is higher than the average consumption, then she should be the one to foot the bill. See whether it works. If she continues to open the air-con you have no choice but to cut off the electricity supply to the aircon in her own room if there is no remote control to the aircon there.
Hi,
I am the same experience with my maid. I placed a small pieces of paper on the air outlet and the paper disappeared when I got back home. I continue did it for a few days and everyday the paper is gone. So I confirm that she did turn on the airconditioner. So I talked to her how expensive the electricity bill are and warned her not to do that anymore.
But the weather is so hot wor .... will be very hot without turning the ac on. I think it's acceptable for the maid to turn on the ac a couple of hours during the day esp. noon time.