function removeSelection(){
    if (window.getSelection) { window.getSelection().removeAllRanges(); }
    else if (document.selection && document.selection.clear)
      document.selection.clear();
}

function killCtrlA(event){
    var event = event || window.event;
    var sender = event.target || event.srcElement;

    if (sender.tagName.match(/INPUT|TEXTAREA/i))
      return;

    var key = event.keyCode || event.which;
    if (event.ctrlKey && key == 'A'.charCodeAt(0))  // 'A'.charCodeAt(0) можно заменить на 65
    {
      removeSelection();

      if (event.preventDefault) 
        event.preventDefault();
      else
        event.returnValue = false;
    }
}

function addHandler(object, event, handler, useCapture) {
      if (object.addEventListener) {
            object.addEventListener(event, handler, useCapture ? useCapture : false);
      } else if (object.attachEvent) {
            object.attachEvent('on' + event, handler);
      } else alert("Add handler is not supported");
}
function removeHandler(object, event, handler) {
      if (object.removeEventListener) {
            object.removeEventListener(event, handler, false);
      } else if (object.detachEvent) {
            object.detachEvent('on' + event, handler);
      } else alert("Remove handler is not supported");
}

function unpreventSelection(element) {
  removeHandler(element, 'mousemove', removeSelection);
  removeHandler(element, 'mousedown', removeSelection);
  removeHandler(element, 'mouseup', removeSelection);
  removeHandler(element, 'keydown', killCtrlA);
  removeHandler(element, 'keyup', killCtrlA);
}

function preventSelection(element) {
  addHandler(element, 'mousemove', removeSelection);
  addHandler(element, 'mousedown', removeSelection);
  addHandler(element, 'mouseup',removeSelection);
  addHandler(element, 'keydown', killCtrlA);
  addHandler(element, 'keyup', killCtrlA);
}