On Ubuntu 16.04 LTS, I successfully used the following to disable suspend:
sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target
And this to re-enable it:
sudo systemctl unmask sleep.target suspend.target hibernate.target hybrid-sleep.target
Mask one or more units, as specified on the command line. This
will link these unit files to /dev/null, making it impossible to
start them. This is a stronger version of disable, since it
prohibits all kinds of activation of the unit, including
enablement and manual activation. Use this option with care. This
honors the --runtime option to only mask temporarily until the
next reboot of the system. The --now option may be used to ensure
that the units are also stopped. This command expects valid unit
names only, it does not accept unit file paths.
Unmask one or more unit files, as specified on the command line.
This will undo the effect of mask. This command expects valid
unit names only, it does not accept unit file paths.