Prophesy not, etc. The men of Anathoth tried first of all to effect their object by threatening. In the name of the Lord should be rather, by the name, etc. The phrase is exactly parallel to Psalms 55:1, "Save me, O God, by thy Name, and judge me by thy strength."
The Name of God is equivalent to his revealed presence or personality. Baal's prophets prophesied "by Baal" (Jeremiah 2:8), i.e. by an impulse thought to proceed from Baal; Jehovah's by the consciousness of his revealed presence.