Guest house tenant running A/C all day. What to do?

[Chris Park]

Guest house utilities are shared with main house under my name.  New tenant and he’s running the A/C all day and night even when he’s not there for long hours at a time

I’ve told him to be mindful and turn it off when he leaves but so far no help.  And he often turns it down to the low 60s

I do have an energy efficient home setup but this is ridiculous.  What can I do outside of charging him for the extra he’s using?