Who breaks the rules of hospitality in the odyssey?

In Homer's "The Odyssey," the Cyclops Polyphemus breaks the rules of hospitality by trapping Odysseus and his men in his cave, refusing to offer them food or safe passage. Instead of treating them as guests, he resorts to violence by eating some of Odysseus's crew. This violation of the sacred Greek custom of xenia, or hospitality, ultimately leads to Polyphemus's downfall when Odysseus outsmarts him and escapes.