the highlight tool is useful, but if you make a mistake you ca't undo it, and once toggled on won't toggle off.
JOHN WALLINGFORD
You can delete the highlight annotation either through the comments panel or select the hand tool, right mouse click the highlight you want to delete and select the "delete" option.
You can toggle the highlight tool off by pressing the Esc key. If you want to remove the last highlight you press ctrl + z (for undo). When the highlight tool is off you can selected any highlighting and remove it by pressing the delete key.