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: