//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
kill -9 44475

sudo lsof -i :<PORT>
kill -9 <PID>

sudo lsof -i :3000

step-1 Enter commond to see running ports & processes
ps -ef
step-2 To stop port and processes
sudo kill {PID}

kill -9 {PID}