What prevents Odysseus from killing the sleeping Cyclops?

"He knows that they cannot move the boulder blocking the doorway.

Explanation:

The tale of Odysseus starts in Homer's epic poem The Iliad, but his second poem, The Odyssey, describes the tale of Odysseus roaming the seas for ten years as he strived to return from the Trojan War. Odysseus was promoted by the goddess Athena for deception and acumen."