Apply a screen protector to the LCD and use a permanent marker to draw the aspect ratio you want to frame for. Relatively inexpensive, easy to add, and completely safe for your screen. You may want to just draw a fairly thin line to mark the safe area; it can sometimes be useful to see the full frame.
I assume you already checked the manual and there's no firmware option.
Well, the easiest way is to hide the excess areas with black electrical tape that you put directly on the screen after measuring the frame height. If needed the tape can be pulled partly off and put back on.
The pricy alternative is a suitable external monitor, called "field monitor".