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