Kill a process using port number

root@FR-121:~# psr
freerad   2744     1  0 Jul01 ?        00:00:00 /usr/sbin/freeradius -i 10.104.99.121 -p 1645
root      6151  4648  0 22:26 pts/4    00:00:00 grep --color=auto radius
root@FR-121:~#
root@FR-121:~#
root@FR-121:~#
root@FR-121:~#
root@FR-121:~# kill $(sudo lsof -t -i:1645)
root@FR-121:~#
root@FR-121:~#
root@FR-121:~# psr
root      6155  4648  0 22:26 pts/4    00:00:00 grep --color=auto radius
root@FR-121:~#
root@FR-121:~#

Source: http://stackoverflow.com/questions/9346211/how-to-kill-a-process-on-a-port-on-ubuntu