Try:
code:
Interop.Call("User32", "SendMessageW", Wnd.Handle, 0x10, 0, 0);
0x10 is WM_CLOSE.
If that doesn't work then try SendMessage instead of SendMessageW.
Sometimes even this method crashes WLM though, but I'm not sure why. It should make the window do exactly what would happen if you pressed the X button.