This example calls WinWindowFromID to get the window handle of the system menu and calls WinSendMsg to send a message to disable the Close menu item.

#define INCL_WINFRAMEMGR   /* for FID_ definitions. */
#define INCL_WINMENUS      /* for MIA_ definitions. */
#define INCL_WINWINDOWMGR
#include <OS2.H>

HWND hwndSysMenu, hwndDlg;

 hwndSysMenu = WinWindowFromID(hwndDlg, FID_SYSMENU);
 WinSendMsg(hwndSysMenu, MM_SETITEMATTR,
     MPFROM2SHORT(SC_CLOSE, TRUE),
     MPFROM2SHORT(MIA_DISABLED, MIA_DISABLED));