That is a mess.
No idea why they would have ran that big a line for just an ice maker.
It could have been done with one flared pipe cap.
Going to have to find the shut off (most likely under the sink)
Set a pan under it and remove those fittings and install a 1/4 turn shut off valve.
It would look a whole lot better with a real ice maker box. http://www.bing.com/images/search?q=...ox&FORM=RESTAB