Kecáš. 
Settings-Extensions/Addons
enable JDShutdown addon.
after that you just have to activate it
menubar-addons-jdshutdown-activate
after download finish, jd will shutdown computer
http://board.jdownloader.org/showthread.php?t=2016 5
http://board.jdownloader.org/showthread.php?t=2016 5