You don’t state your location, which is relevant to the question.
You can add an ordinary electric meter to your guest room circuit, maybe just the line going to the A/C. This is a low tech solution, and won’t allow you to control anything, but it will tell you whether your guests are leaving the A/C on when they go out. The meter displays a blinking red light if there is power consumption, and the heavier the power consumption. the more frequently it will blink. At which point you can either go in there and turn it off, or remind them to do so.
I’m located in India. Smart devices are hard to find here. This may also be the case in other places. If you happen to be located in the North America, Australia, New Zealand, Japan, Western Europe or the UK, you probably have better options available to you.
Another option that has been discussed here is to have some form of card switch or key switch in the room that forces guests to turn off the electricity when they leave. But this only really works if the door locks uses the same card or key, and you only give them one card or key. Then they have to take out the card or key to lock the door, forcing them to turn the electricity off.