Fruitflys are unique in that they can breed in hidden locations off the food and garbage. When the larvae get their full, they'll migrate away from the garbage and find nooks and crevices where they can pupate (change into adults). That means even though the garbage isn't there, the pupae will be able to hatch out adults and that will always be true until you use something to kill the eggs.

Killing the eggs requires the use of Gentrol Aerosol as explained in our Fruitfly Control article. But keep in mind the important place to apply it is not the garbage pail itself but instead to all the areas around the pail. I would focus in on the inside of the cabinet where you keep the pail. I'd also treat the surrounding spaces. All it takes is a few making it far enough away to where your treatments haven't reached. These few could easily continue the cycle and I'm sure that's been the case. Get the Gentrol applied and you should be able to break the cycle once and for all.