While it sounds like a great idea and the perfect way to go it too much of a system shock for most people.
Fast food often will contain ingredients that the body gets used too, high fructose corn syrup, MSG, and food dyes.
Not only will your system be craving these items, the body will be going through withdrawl at the same time.
This approach weakens will power and sets a person up for binge eating later on.
it is best to have a slower approach to healthy foods, remember that weight problems happen slowly over a long period of time, the solution is going to take some time as well.