It should not be banned but since it's for the sake of argument, well, let's just put it this way....
1) With fastfoods around, there are more disposable garbage that are so hazardous not only to the people but to the environment as a whole.
2) With this kind of food, nobody is so sure as to how clean the food was processed.
3) The reason why people are getting fatter and fatter, that's due to the MSG (monosodium glutamate) which are hazardous to our health.
4) The parents are forced to become timid in preparing for their daily meals.
5) Mothers who are expected to have knowledge of preparing food for the family no longer make efforts in cooking.
6) They have the tendency to run out of cash coz they could no longer manage to budget their income.
I think these are enough reasons for banning fastfoods as starting points.
Good luck and be sure to let me know what happened to your debate. I hope you don't mind :)
Take care and GOD BLESS!