ubuntu has kill command. While running
webrick server, in project directory within location
APP_DIR/tmp/pids/server.pid there will be all Process Ids saved.
You just need to open the file, you will find the Process Id of currently running server. Now you can use the following command to kill the process
$ kill [pid] # Example kill 8123
For Windows user, you can use the following command
taskkill /PID 8436