@jcboy,
Or there might be a Rental Board for the City or State - in BC, Canada (and presumably across the country), there is a Rentalsman Office (or whatever PC name they have now) which provides advice to both landlords and tenants, lists the responsibilities of each, provides free lease forms (etc), and even calculates the interest on damage/security deposits for you. I'm sure there is information online for where you live that you can access.
Good luck. It sounds like a stressful situation.
If you cut the gas (and did you also say electricity?), how will you have any heat, lights, tv, internet, etc.?
Also, as a previous landlord myself, I would ask the remaining tenant to pay and they could pursue the offender in Small Claims Court or get another roommate. It wouldn't matter if one had moved out or died... the rent still has to be paid.