Const WM_CLOSE = &H10
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, _
        ByVal lpWindowName As String) As Long
Private Declare Function PostMessage Lib "user32" Alias "PostMessageA" (ByVal hwnd As Long, _
        ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long

Private Sub Command1_Click()
    Dim WinWnd As Long, Ret As String
    Ret = InputBox("Введите имя окна (титул):" )
    WinWnd = FindWindow(vbNullString, Ret)
    If WinWnd =  0  Then MsgBox "Не найдено": Exit Sub
    MsgBox "Найдено"
    'Закрываем то что найдено
    PostMessage WinWnd, WM_CLOSE,  0 &,  0 &
End Sub