Sprinkle an emetic (a vomit-inducing drug) into foods that you want to stop eating, such as chocolate. It is well-known that nausea causes a long-lasting aversion to the food preceding it. (For instance, this is a problem for chemotherapy patients - the drug therapy causes nausea, which they then associate with food.)
I haven't tried any of this, but I'd be very surprised if this wasn't an easy, long-term solution to the problem of people wanting to eat food that they don't want to want to eat.
Maybe this could even be extended to non-food addictions, such as video games or mindless internet browsing. One person I know quit smoking cold turkey this way (by throwing up after smoking a cigarette, not with an emetic).
I would personally recommend against training your body out of finding particular foods pleasurable. Instead, I would recommend exploring alternative food combinations that satiate the same craving.
I.e., expand your palette rather than restrict it.
Also, mindfulness meditation can be useful here. I have a reasonable amount of anecdotal evidence (p ~= 0.7) that a lot of overeating problems center around focussing on the oral aspects of digestion rather than the gastrointestinal.
Remember that your stomach has enough neurons to make an entire second brain - ...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.