Drinking a lot of water at once in order to fill up your stomach is terrible advice. While it might work, it could also have some adverse effects on your overall health and might cause you to develop mineral deficiencies.
Filling up on healthy foods at the onset of fast food cravings on the other hand, would be a good solution, even in the long term, but Im sure it wont be easy and require lots of willpower in order to work.
I had a bad sugar habit almost all my life, a habit which I have changed over the last couple of years, but I still crave it big time if Im high and its around, to the point where I can´t keep away, and dont stop eating it until its all gone, leaving me with horrible side effects. I find this sugar shit to be more addicting than weed, MDMA and opiates in my case.
The great thing is, and Im sure this goes for fast food too, is that, the less often you eat it, the less you´ll crave it - with time! A long fucking time, but eventually, things starts to reverse and you start craving healthy foods instead. It might take several years of healthy living but it can be done, Im well on my way and you can do it too.