Flash MX 2004.
element.locked
Property; a Boolean value: true if the element is locked; false otherwise. If the value of element.elementType is "shape", this property is ignored.
The following example locks the first element in the first frame, top layer:
// Similar to Modify > Arrange > Lock:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0].locked = true;