Positive method would be to tell your dog no! To any food that is not in her bowl, then reward her with a higher price food prize. Repeat this method daily, start off as you would with obedience in a non distracting environment such as your house, them gradually introduce the same scenarios in more difficult environments.
Non positive method- use a shock collar give her a light zap to any interest shown in food not in her bowl. Start in house then go on to more difficult environments.
Do this for a week, and them I'll explain the next step I take. This includes doing the exercises while you are out of sight, not at home, with other people trying to bait your dogs etc.
The key is knowing individual dogs and tweaking the exercise to accommodate problems or successes you get with them while teaching the desired refusal behavior
When you say she has a low threshold is it hard to get her into a certain drive? Or is it an environmental thing where she is seems unaware of danger and excepting of anything in new environments?