Можно и без фокуса:

Public Const SW_HIDE = 0
Public Const SW_SHOWNORMAL = 1
Public Const SW_NORMAL = 1
Public Const SW_SHOWMINIMIZED = 2
Public Const SW_SHOWMAXIMIZED = 3
Public Const SW_MAXIMIZE = 3
Public Const SW_SHOWNOACTIVATE = 4
Public Const SW_SHOW = 5
Public Const SW_MINIMIZE = 6
Public Const SW_SHOWMINNOACTIVE = 7
Public Const SW_SHOWNA = 8
Public Const SW_RESTORE = 9
Public Const SW_SHOWDEFAULT = 10
Public Const SW_MAX = 10
Public Const WM_CLOSE = &H10
Public Const INFINITE = &HFFFFFFFF

Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
( _
ByVal hwnd As Long, _
ByVal wMsg As Long, _
ByVal wParam As Long, _
lParam As Any _
) _
As Long

Call SendMessage(Me.hwnd, WM_CLOSE, 0&, 0&)

Где Me - ссылка на форму(окно) которую хочеш закрыть