It shall not return any more — i.e., until it has fully accomplished its purpose. Other judgments upon Israel had been arrested in mercy — the sword had been returned to the scabbard while its work was still incomplete. This will go on to the end.
Related Commentaries of Ezekiel 21:5
Ezekiel 21:5
5 That all flesh may know that I the LORD have drawn forth my sword out of his sheath: it shall not return any more.