public bool Locked {get; set;}

{
  // The Locked property is displayed in the TOC as a lock symbol next
  // to each element.  If locked the element can't be selected in the layout
  // using the graphic selection tools.
  //Note: Must be on QueuedTask.Run

  //Reference an element 
  if (element != null)
  {
    // Modify the Locked property via the CIM
    CIMElement CIMElement = element.GetDefinition() as CIMElement;
    CIMElement.Locked = true;
    element.SetDefinition(CIMElement);
  }
}