How can I close an application?

You can use the FindWindow and PostMessage API functions to find a window and then close it. This example show how you can close down a Window with a caption of "Calculator".

Dim winHwnd As Long
Dim RetVal As Long
winHwnd = FindWindow(vbNullString, "Calculator")
Debug.Print winHwnd
If winHwnd <> 0 Then
    RetVal = PostMessage(winHwnd, WM_CLOSE, 0&, 0&)
    If RetVal = 0 Then
        MsgBox "Error posting message."
    End If
Else
    MsgBox "The Calculator is not open."
End If

For this code to work, you must have declared the API functions in a module in your project. You must put the following in the declarations section of the module.

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