Hombre, no te vuelvas loco (broma) aquí te dejo un código que lo resuelve con unas llamadas a la API del Windows:
Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, _
ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Private Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" (ByVal hwnd As Long, _
ByVal nIndex As Long) As Long
Private Declare Function GetSystemMenu Lib "user32" (ByVal hwnd As Long, ByVal bRevert As Long) As Long
Private Declare Function RemoveMenu Lib "user32" (ByVal hMenu As Long, ByVal nPosition As Long, _
ByVal wFlags As Long) As Long
Private Sub MDIForm_Load()
Dim lRet As Long
Dim hSysmenu As Long
lRet = GetWindowLong(Me.hwnd, (-16))
lRet = lRet And Not &H20000 'Desactiva el botón de Minimizar
lRet = SetWindowLong(Me.hwnd, (-16), lRet)
hSysmenu = GetSystemMenu(Me.hwnd, 0)
RemoveMenu hSysmenu, 3, &H400 ' elimina el menú "minimizar" del MDI
RemoveMenu hSysmenu, 2, &H400 ' elimina el menú "tamaño" del MDI
End Sub
Saludos.