Tenants vs Owner paying for heating oil

[Tyrel Ferris]

We have detached 4 Plex in Alaska. We purchased the beginning of the year. The leases are month to month, and it included heating oil when we bought the property. 

How can we go about having the tenants pay for their own heating oil? 

They're not even trying to conserve it. It's about $1500-$2000 a month because they're just burning it like crazy since we pay for it.

What's the best way to tell them they're going to have to start paying for it themselves?