Killing the wagtail server will stop it. Search the PID for your wagtail:
ps aux | grep "manage.py runserver"
You'll get a response like this:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
[...]
user 38 0.0 0.0 0 0 ? S 2433 0:00 python manage.py runserver 0:8000
Then kill it by using:
# Terminate running process
kill -9 38
Then you can restart as you want