Veja o exemplo, dá certo em mdi e child.
Option Explicit
Private Declare Function DeleteMenu Lib "user32" (ByVal hMenu As Long, ByVal nPosition As Long, ByVal wFlags As Long) As Long
Private Declare Function GetSystemMenu Lib "user32" (ByVal Hwnd As Long, ByVal bRevert As Long) As Long
Private Const MF_BYPOSITION = &H400&
Private Sub RemoveMenus()
Dim hMenu As Long
hMenu = GetSystemMenu(Hwnd, False)
DeleteMenu hMenu, 6, MF_BYPOSITION
End Sub
Private Sub Form_Load()
RemoveMenus
End Sub
vlu//