XPLM_API int        XPLMUnshareData(
                         const char *         inDataName,    
                         XPLMDataTypeID       inDataType,    
                         XPLMDataChanged_f    inNotificationFunc,    
                         void *               inNotificationRefcon);    

This routine removes your notification function for shared data. Call it when done with the data to stop receiving change notifications. Arguments must match XPLMShareData. The actual memory will not necessarily be freed, since other plug-ins could be using it.