To remove the handler from the event, use RemoveFromEvent.
example: function SiteLibrary_DefaultPage$example(viewer) { // The method to call on image changed var imageChangedHandler = null; imageChangedHandler = function(sender, e) { alert("Image has changed"); // Remove the event Leadtools.LeadEventArgs.removeFromEvent(viewer, "imageChanged", imageChangedHandler); }; // Add a handler to the viewer.ImageChanged event Leadtools.LeadEventArgs.addToEvent(viewer, "imageChanged", imageChangedHandler); // Set an image into the viewer and notice our alert viewer.set_imageUrl("Images/24.png"); },