How to kill a process that never dies?
init: logstash main process (2546) killed by KILL signal
Actually your process does stop here.
init: logstash main process ended, respawning
A new logstash process is started by init to replace it.
That is also shows which control process is responsible for restarting logstash: init. (On RHEL 6 and CentOS that is Upstart) Your process most likely gets started from either/etc/inittab or a drop-in file in /etc/init/logstash.conf (or similar) and should be controlled with the apropiate tool, initctl and not with kill.
Try initctl list to see if logstash is there.
Then initctl stop logstash will stop it.
Editing or removing the conf file in /etc/init will allow you to disable it persistently.
You might even be able to control the job with the service and chkconfig commands.