Can you knock someone out by pinching their jugular vein in their neck?

Only if Spock: This maneuver was popularized by leonard nimoy as spock on star trek. It is not possible to do this in reality.

Please don't rry: Blunt force to the neck can cause a variety of life threatening injuries, and syncope (passing out) may be the least of your worries. Carotid dissection, esophageal injury, tracheal collapse, or a head injury are all possible -- please do not punch people in the neck.