When stopping a currently-running server

Check the process ID of a currently-running server

$ cat tmp/pids/server.pid
26711

Kill that process

$ kill -9 26711

When a process appears to be frozen

Check all the processed on your system.

$ ps aux

Check all the processed with the name spring

$ ps aux | grep spring
masa            28136   0.0  0.0  2434840    776 s001  S+    3:43PM   0:00.00 grep spring

Try to stop the spring server

$ spring stop

Kill all the processes of spring

$ pkill -9 -f spring

Kill an individual process

$ kill -9 <pid>