I have pretty much eaten fast food at the very least 3 days out of the week although there was a 7 month stent in which i ate every day. I burn alot of calories at my job so ive been lucky to not have gained weight.
The reason i don't stop is that i really don't want too, i'll stop for a while but i end up eating fast food again. Soon i wont have a choice since i won't be able to afford it any more.
Best advice is to go cold turkey (they don't serve turkey there do they?) and bring you own lunch. i would start with getting canned soup, its pretty cheap compared to proportions of other food and is pretty filling and tastes good.
Eat before you go in, take fruit with you, have some self control.
eating before going in is a good idea. its less an issue with self control as it is an issue with motivation as ive personally discovered. Its not that you have no control, its that you do have control and while you know its bad you still want it. Tastes good > healthy is the scale of the motivation. At least in my case.