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

#define INCL_WINMENUS
#define INCL_WINMESSAGEMGR
#define INCL_WINFRAMEMGR
#include <OS2.H>
HWND hwndDlg;
HWND hwndSysMenu;

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