I actually had a similar situation this week while sitting in a drive thru. The engine was warm, and no matter what I did the gas engine would not turn off. What I did to turn it off was to turn the climate control OFF. It was roughly 30 degrees (F) outside, so I had the heat blowing. When I turned the heat back on, the engine turned back on. I turned the climate control off/on again 2 more times just to see if it was a fluke- but no... Climate control on = gas engine running. Climate control off = no gas engine.

This got me thinking/experimenting: After starting the car in the morning, even when it's cold outside, I have been able to stop the gas engine from running at stoplights if I simply turned the climate control completely off.

This works even when the engine is not at normal operating temperature- but it does have to be out of the blue zone.