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sudo gedit /etc/polkit-1/localauthority/50-local.d/disable-suspend.pkla
[Disable suspend]
Identity=unix-user:*
Action=org.freedesktop.upower.suspend;org.freedesktop.upower.hibernate
ResultActive=no
ResultAny=no

Reference

http://ohioloco.ubuntuforums.org/showthread.php?t=1305081&page=2