//To find the process id (PID) associated with the port

⇒ lsof -i tcp:3000
COMMAND PID   USER  FD  TYPE DEVICE             SIZE/OFF NODE NAME
node    44475 chen5 31u IPv4 0x8b1721168764e4bf 0t0 TCP *:strexec-s (LISTEN)

//Then to kill the process (Use -9 option to make sure the process dies immediately)

kill -9 44475