The component you actually need to be dealing with is the JInternalFrame which contains the JPanel you mentioned above. This should have a number of functions to enable/disable the actions associated with the min/max/close buttons (E.g.: setMaximizable(bool enabled)
).
I do not know if this would hide the buttons or merely disable them, so you may have to use some variant of the trick mentioned by R.J - manually removing the buttons.