If not use this command to kill the port on which the your app is running,

   fuser -n tcp -k (port number)

ps aux | grep rails

kill -9 pid

=> Booting WEBrick
=> Rails 4.0.0 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
[2014-04-08 21:14:42] INFO  WEBrick 1.3.1
[2014-04-08 21:14:42] INFO  ruby 2.0.0 (2013-11-22) [x86_64-linux]
[2014-04-08 21:14:42] INFO  WEBrick::HTTPServer#start: pid=2677 port=3000

kill 8976