The best way to stop the wastage of food is to always clear our the fridge. Usually those who love to waste food always have a full fridge as they do not know how to manage food.
At parties, I don't think the wastage can be reduced. It can be minimized.
Before any party find out the number of people who are going on this party.
Then one day before having the party, get the right amount of people. It will drastically reduce the total number of people who may not have showed up.