Uninstalling

Warnings:

If your server federates with others, make sure to leave any federated rooms before nuking your Matrix server's data. Otherwise, the next time you set up a Matrix server for this domain (regardless of the installation method you use), you'll encounter trouble federating.
If you have some trouble with your installation, you can just re-run the playbook and it will try to set things up again. Uninstalling and then installing anew rarely solves anything.

Uninstalling using a script

Installing places a /usr/local/bin/matrix-remove-all script on the server.

You can run it to to have it uninstall things for you automatically (see below). Use with caution!

If you prefer to uninstall manually, run these commands (most are meant to be executed on the Matrix server itself):

ensure all Matrix services are stopped: ansible-playbook -i inventory/hosts setup.yml --tags=stop (if you can't get Ansible working to run this command, you can run systemctl stop 'matrix*' manually on the server)
delete the Matrix-related systemd .service and .timer files (rm -f /etc/systemd/system/matrix*.{service,timer}) and reload systemd (systemctl daemon-reload)
delete some helper scripts (rm -f /usr/local/bin/matrix*)
delete some cached Docker images (docker system prune -a) or just delete them all (docker rmi $(docker images -aq))
delete the Docker networks: docker network rm matrix matrix-coturn (might have been deleted already if you ran the docker system prune command)
uninstall Docker itself, if necessary
delete the /matrix directory (rm -rf /matrix)