We tell them that their friends can't eat our food without permission. There had been a couple times when my son's teenaged friends ate some dinner of ours and my own children went without dinner because it was gone by the time they got to the kitchen!
We had a family meeting about that. Our food smells so good sometimes, the friends told my son man, that smells so good. Well, at first, my teenaged son would offer the food to his friends before everyone had eaten (we don't all sit at the table. Everyone just gets their food and eats wherever). We made a rule that you can't offer dinner to your friends until everyone in our family has eaten. Afterwards, i might offer the friends the leftovers. Especially food that doesn't taste better the second day, like seafood.

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It got to where they just bring their own bag food over and put it in my son's minifridge. Especially as they got older and all had their own cars. We put a microwave down in the basement, and they would microwave dinners for themselves . This is because as gamers they would come over for weeks at a time sometimes and lan and sleep on our couches, floors, etc. Kids that stay for weeks at a time, I fed more often when I had the money. We loved to entertain during a lan. But it got to where we couldn't afford it as much as we used to. Sometimes we have extra money and we make cookies and offer. When I used to have extra money I would stock the mini fridge with all kinds of drinks, but those days are long gone. We just can't do it anymore. When we offer food, it is ok, and it is special. I just apologized when I brought my son his food down to his room while he was gaming, that I was sorry that I don't have enough to feed everybody....so basically all the friends respect that. The ones who don't, I can count on one hand....and I don't like. I love it when we do have extra money to cook for the friends, and I do have favorite friends that we just love to cook for, the ones who really appreciate it, and say "I look forward to coming over to your house for the awesome food". It's just that right now I can't afford to do that, so it is a real bummer.
I told my kids....just look at that kid that comes over and takes food w/o asking....its rude and its disrespectful...he was 16! He went into our fridge and took food w/o asking. that is all I remember about that kid and my kids know

: how much it irks me.