Que eu saiba, tirando o botão minimizar, já não deixa o form ser minimizado, nem mesmo com as teclas de atalho.

Em um módulo:

Public Declare Function SetWindowLong Lib "user32" _
        Alias "SetWindowLongA" (ByVal hwnd As Long, _
        ByVal nIndex As Long, ByVal dwNewLong As _
        Long) As Long
Public Declare Function GetWindowLong Lib "user32" _
        Alias "GetWindowLongA" (ByVal hwnd As Long, _
        ByVal nIndex As Long) As Long

Public Const WS_MINIMIZEBOX As Long = &H20000
Public Const WS_MAXIMIZEBOX As Long = &H10000
Public Const GWL_STYLE As Long = (-16)

 

No Form:

  Dim lWnd As Long
  lWnd = GetWindowLong(Me.hwnd, GWL_STYLE)

  'Tira o botão de minimizar
  lWnd = lWnd And Not (WS_MINIMIZEBOX)

  'Tira o botão de maximizar
  lWnd = lWnd And Not (WS_MAXIMIZEBOX)

  lWnd = SetWindowLong(Me.hwnd, GWL_STYLE, lWnd)