My guest leaves the heating on with the window open

A guest pays nothing extra if he heats with the window open, so he feels no deterrent whatsoever, and a note by the thermostat usually doesn't help because there's no consequence attached. You only see the costs back in the annual bill, together with all other consumption, which means you can't address this per guest. With a separate credit balance that the guest sees dwindling on his phone, behavior changes without you having to say anything: as soon as the window stays open, he notices it directly himself in his consumption.