to open a conversation window with someone:
code:
Messenger.OpenChat("email@provider.com");
and to close it, you need to get the handle of the chat window and then send the WM_CLOSE message to the window:
code:
var WM_CLOSE = 0x0010;
Interop.Call("user32", "SendMessageW", hWnd, WM_CLOSE, 0, 0);