You could use the FindWindow and SendMessage API calls
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
hWnd = FindWindow(vbNullString, MsgTitle)
Call SendMessage(hWnd, WM_CLOSE, 0, ByVal 0&)