To disable automatic suspend, but leave manual suspend working in Ubuntu 14.04, create the text file:

/etc/polkit-1/localauthority/50-local.d/com.ubuntu.disable-suspend.pkla

with contents:

[Disable suspend (upower)]
Identity=unix-user:*
Action=org.freedesktop.upower.suspend
ResultActive=no
ResultInactive=no
ResultAny=no

[Disable suspend (logind)]
Identity=unix-user:*
Action=org.freedesktop.login1.suspend
ResultActive=yes
ResultInactive=no
ResultAny=yes

[Disable suspend when others are logged in (logind)]
Identity=unix-user:*
Action=org.freedesktop.login1.suspend-multiple-sessions
ResultActive=yes
ResultInactive=no
ResultAny=yes