I have tried this, but no use. So in the command below, you can see that mosquitto is restarted again with different PID, I have also tried pkill and killall but the behaviour is same.
$ ps -A | grep mosquitto
39310 ttys001 0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn mosquitto
$ brew services stop mosquitto
Error: Service `mosquitto` is not started.
$ ps -A | grep mosquitto
39630 ttys001 0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn mosquitto