Quit it all cold turkey! Or let yourself eat only one or two junk foods that aren't that bad.
What really helped me was planning out my meals the night before. What also helps is when I am at work, what I brought is all I get to eat. And if you plan your small meals/snacks so that you can have one every couple of hours, you barely have time to even THINK about eating junk, cause you are never hungry. Try eating healthy foods that you love, or finding new ones! I wa sa junk food junkie before going on the IBS diet, just becauase I didn't know how good healthy stuff tasted! Try making some smoothies with soymilk and strawberries and bananas... try different fruits that you may not have before. Last year was the first time I tried a mango, now it is my favorite food ever! Once you get away from junk food long enough, it stops tasting as good. Now if I eat really processed snack foods or sugary foods I go "Ack, my memory of that is better than how it tastes!" learn some cooking techniques, make all of your healthy snacks/meals good and you will never think about your oreos again