Dear Varsha,
First, if you are giving unlimited food then stop it (portion the food items per plate), if they want extra they can buy more.
Secondly if they eat too much then they will not be able to work. Will go to sleep or will become lazy.
Portion control is the best way you can control wastage. Also then it will help in production forecast.
I have worked in big canteens for almost 25 years.
What ever suggestions are given by others are also good and slowly implement them and make posters of real photographs were human beings are fighting for food daily. Looking at photos there conscious will warn them.
all the best. When you achieve your target. Inform us. (gautamshastry@gmail.com)
Gautam Shastry