The WM_DDE_UNADVISE message instructs a server to stop posting WM_DDE_DATA messages when a data item has changed. The low word of lParam is either the data format or 0, indicating all data formats. The high word of lParam is either the item ATOM or NULL to indicate all items.
DDEPOP handles the WM_DDE_UNADVISE message by setting the appropriate fAdvise fields of the POPADVISE structure to FALSE, and then acknowledging with a positive or negative WM_DDE_ACK message.