my_handle := FindWindows( nil ,'Caption');
PostMessage(...,WM_QUIT);

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 Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
Const SW_SHOWNORMAL =  1 
Const WM_CLOSE = &H10
Const gcClassnameMSWord = "OpusApp"
Const gcClassnameMSExcel = "XLMAIN"
Const gcClassnameMSIExplorer = "IEFrame"
Const gcClassnameMSVBasic = "wndclass_desked_gsk"
Const gcClassnameNotePad = "Notepad"
Const gcClassnameMyVBApp = "ThunderForm"
Private Sub Form_Load()
    'KPD-Team 1998
    'URL: http://www.allapi.net/
    'E-Mail: KPDTeam@Allapi.net
    Dim WinWnd As Long, Ret As String, RetVal As Long, lpClassName As String
    'Ask for a Window title
    Ret = InputBox("Enter the exact window title:" + Chr$( 13 ) + Chr$( 10 ) + "Note: must be an exact match")
    'Search the window
    WinWnd = FindWindow(vbNullString, Ret)
    If WinWnd =  0  Then MsgBox "Couldn't find the window ...": Exit Sub
    'Show the window
    ShowWindow WinWnd, SW_SHOWNORMAL
    'Create a buffer
    lpClassName = Space( 256 )
    'retrieve the class name
    RetVal = GetClassName(WinWnd, lpClassName,  256 )
    'Show the classname
    MsgBox "Classname: " + Left$(lpClassName, RetVal)
    'Post a message to the window to close itself
    PostMessage WinWnd, WM_CLOSE,  0 &,  0 &
End Sub