What do you think about someone eating my pizza?
Wow, will you invite them back over for dinner again?
No, I find it very rude for them to help themselves to food I didn't offer them.
That's the way I would feel too. It's beyond inexcusable to eat someone else's food in their own home, especially if it's food you didn't offer them.
Yeah, I want to tell my other friends to warn them about what they might do to them as well.
It's really rude when someone comes to your home and eats food that they weren't allowed to eat. I think you could have them back over, but be very clear about what they are allowed to eat so that there's no misunderstandings in the future.