Could you install a timer that automatically turns it off instead? It might go over better with the roommate.
I'm interested in this too, cause, uh, my roommate does the same thing. Only difference is our oven is gas. I'm more worried about safety than cost at this point. At least twice it's been left on for 24+ hours until I discovered it. He just says "oops." Leaves space heaters on too. And lights, but I've given up worrying about the lights. Financially I'm still saving way more money by having a roommate than he's costing in additional utilities.
What I've done so far is get renter's insurance with a million dollar liability coverage.