var hMenu = GetSystemMenu(hWnd, false);
if (hMenu == IntPtr.Zero) return;
EnableMenuItem(hMenu, SC_CLOSE, MF_BYCOMMAND | MF_GRAYED);
var cnt = GetMenuItemCount(hMenu);
RemoveMenu(hMenu, cnt - 4, MF_BYPOSITION | MF_DISABLED);
DrawMenuBar(hWnd);

....
[DllImport("user32.dll")]
static extern bool EnableMenuItem(IntPtr hMenu, uint uIdEnableItem, uint uEnable);
[DllImport("user32.dll", EntryPoint = "GetMenuItemCount")]
public static extern int GetMenuItemCount(IntPtr hmenu);
[DllImport("user32.dll", EntryPoint = "RemoveMenu")]
public static extern int RemoveMenu(IntPtr hmenu, int npos, uint wflags);
[DllImport("user32.dll")]
public static extern bool DrawMenuBar(IntPtr hWnd);