@Ann44, one option might be to require a hefty Security Deposit (which you should do in any case), and state in your House Rules that if AC is used to an amount exceeding $XXX per day, a claim for the extra cost will be made against the Security Deposit.
Perhaps that will motivate guests to turn off the AC before they go out.
UPDATE: I've noted that your House Rules are not firm enough. I recommend that you be very specific about what is expected of guests. They need to know the clear boundaries, such as not leaving the AC on when they go out.