Disabling the Maximize menu button

I need to disable the Maximize button on the main form's system menu. I
have tried the following code and it does not seem to do anything?

   h_Menu := GetSystemMenu(Self.Handle, FALSE);
   EnableMenuItem(h_Menu, SC_MAXIMIZE, MF_BYCOMMAND or MF_GRAYED);

I have used this code (changing SC_MAXIMIZE to SC_CLOSE) and it works to
disable the Close button. Any Ideas?
Thanks