procedure TForm1.FormCreate(Sender: TObject);
var
  mi : TMenuItemInfo;
  hm: HMENU;
begin
  hm := GetSystemMenu(Handle, false);
  mi.cbSize := sizeof(TMenuiteminfo);
  mi.fMask := MIIM_STATE;
  GetMenuItemInfo(hm,SC_CLOSE,false,mi);
  mi.fState := MFS_DISABLED;
  SetMenuItemInfo(hm,SC_CLOSE,false,mi);
end;

var
  Flag: UINT;
  AppSysMenu: THandle;
begin
  AppSysMenu:=GetSystemMenu(Handle,False);
  Flag:=MF_GRAYED;
  EnableMenuItem(AppSysMenu,SC_CLOSE,MF_BYCOMMAND or Flag);
end;

Procedure DisabledTombolClose;
var
  MenuHandle: Integer;
begin
  MenuHandle := GetSystemMenu(Handle, False);
  if (MenuHandle <> 0) then
    ModifyMenu(MenuHandle, SC_CLOSE, MF_BYCOMMAND or
               MF_GRAYED or MF_DISABLED, SC_CLOSE, 'Close');

procedure TForm1.FormCloseQuery(Sender:TObject; var CanClose:Boolean);
begin
 CanClose := False;
end;

@chipmunk: ..disable tombol close tidak mencegah closing misal dg ALT-F4, tp bisa ditrap pada OnCloseQuery()... utk men-disabled aja (system menu jg akan sinkron) coba code ini di OnCreate()

procedure TForm1.FormCreate(Sender: TObject);
var
  mi : TMenuItemInfo;
  hm: HMENU;
begin
  hm := GetSystemMenu(Handle, false);
  mi.cbSize := sizeof(TMenuiteminfo);
  mi.fMask := MIIM_STATE;
  GetMenuItemInfo(hm,SC_CLOSE,false,mi);
  mi.fState := MFS_DISABLED;
  SetMenuItemInfo(hm,SC_CLOSE,false,mi);
end;

- chipmunk -