In https://docs.zammad.org/en/latest/install-elasticsearch.html I don’t see how to disable, or better remove, Elasticsearch.

Using version:

zammad:
  Installed: 2.3.0-1518538476.8684b1e4.xenial
  Candidate: 2.3.0-1518538476.8684b1e4.xenial
  Version table:
 *** 2.3.0-1518538476.8684b1e4.xenial 500
        500 https://dl.packager.io/srv/deb/zammad/zammad/stable/ubuntu 16.04/main amd64 Packages
        100 /var/lib/dpkg/status
     2.3.0-1518189665.73199b9b.xenial 500
        500 https://dl.packager.io/srv/deb/zammad/zammad/stable/ubuntu 16.04/main amd64 Packages
[...]
First I check status:
systemctl status elasticsearch:
● elasticsearch.service - Elasticsearch
   Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; disabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Fri 2018-02-16 11:04:06 CET; 8min ago
     Docs: http://www.elastic.co
 Main PID: 1258 (code=exited, status=1/FAILURE)
Ok, it’s not running/fails to run (2GB RAM is not enough), so I want to disable it: systemctl disable elasticsearch
Then I check again:
systemctl status elasticsearch:
● elasticsearch.service - Elasticsearch
   Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; disabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Fri 2018-02-16 11:04:06 CET; 8min ago
     Docs: http://www.elastic.co
 Main PID: 1258 (code=exited, status=1/FAILURE)
Still tries to start.
# systemctl disable elasticsearch
Synchronizing state of elasticsearch.service with SysV init with /lib/systemd/systemd-sysv-install...
Executing /lib/systemd/systemd-sysv-install disable elasticsearch
insserv: warning: current start runlevel(s) (empty) of script `elasticsearch' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `elasticsearch' overrides LSB defaults (0 1 6).
insserv: warning: current start runlevel(s) (empty) of script `elasticsearch' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `elasticsearch' overrides LSB defaults (0 1 6).
# systemctl status elasticsearch
● elasticsearch.service - Elasticsearch
   Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; disabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Fri 2018-02-16 11:04:06 CET; 34min ago
     Docs: http://www.elastic.co
 Main PID: 1258 (code=exited, status=1/FAILURE)
[...]
It unfortunately still is loaded.
              
I could try https://docs.zammad.org/en/latest/install-elasticsearch.html#ubuntu-16-04 in reverse. Hope Zammad will still work.
Ok, did:

# rm /etc/apt/sources.list.d/elastic-5.x.list

# apt-key list

# apt-key del D88E42B4

# apt remove elasticsearch
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libevent-core-2.0-5 libevent-extra-2.0-5
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  elasticsearch zammad
0 upgraded, 0 newly installed, 2 to remove and 0 not upgraded.
After this operation, 297 MB disk space will be freed.
Do you want to continue? [Y/n] ^C
It shows that zammad as well will be removed:
#     /etc/init.d/elasticsearch stop

[ ok ] Stopping elasticsearch (via systemctl): elasticsearch.service.

#     ps auxww |grep elasticsearch

root      4750  0.0  0.0  12944   972 pts/0    S+   12:07   0:00 grep --color=auto elasticsearch

#     update-rc.d -f elasticsearch remove

But still loads:

# systemctl status elasticsearch

● elasticsearch.service - Elasticsearch

Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; disabled; vendor preset: enabled)

Active: failed (Result: exit-code) since Fri 2018-02-16 11:04:06 CET; 1h 3min ago

Docs: http://www.elastic.co

Main PID: 1258 (code=exited, status=1/FAILURE)
I’m doing a reboot now and see if it starts.
After reboot it shows:

# systemctl status elasticsearch
● elasticsearch.service - Elasticsearch

Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; disabled; vendor preset: enabled)

Active: inactive (dead)

Docs: http://www.elastic.co
It’s now inactive but still loaded?