Script for turn off ligatures from overall text of an document

Try this:

/**
 * Turn off ligatures in active document.
 * @7111211 m1b
 */
(function () {

    var doc = app.activeDocument,
        textFrames = doc.textFrames;

    for (var i = 0; i < textFrames.length; i++) {
        textFrames[i].textRange.ligature = false;
        textFrames[i].textRange.contextualLigature = false;
        textFrames[i].textRange.discretionaryLigature = false;
    }

})();