When Smile deletes an object (for instance, when the user closes a window), Smile sends a delete event to the object's script. You handle delete with a handler such as:

on delete the_object
    -- do whatever suitable
    continue delete the_object
end delete

the_object contains a reference to the object being deleted, the owner of the script. The continue command is required to have Smile actually dispose of the object: you can choose not to send it - for instance if some condition has failed.