Assuming a UNIX-like system:
$ ps aux | grep elixir
to find the process-id of the OS-process that runs the Erlang VM that has loaded elixir and your phoenix application, and then
kill number
to stop it. number is the number in the second column of the output of ps aux (the first column containing the name of the unix-user that owns, i.e. started, the process).
Afterwards you can try ps aux | grep elixir again to make sure it is no longer there 
.
In an actual production environment, it is advisable that you instead set up e.g. a systemd entry that manages the process, which makes it a lot easier to start/stop/restart/autostart-at-boot and monitor it.
