To remove the buttons you are the top-right of portlet simply edit the css file used according to the buttons you want to delete.
I make the example for the Maximize button but the same is true for both the button and the Minimize button Normal.
.portlet-mode-maximized {
display:none;
visibility:hidden;
position:absolute;
left:-9999em;
}
Simply replace the ".portlet-mode-maximized" with :
".portlet-mode-minimized" and ".portlet-mode-normal"