Find the process ID (PID) for the port (e.g.: 8080)
On Windows:
netstat -ao | find "8080"
Other Platforms other than windows :
lsof -i:8080
Kill the process ID you found (e.g.: 20712)
On Windows:
Taskkill /PID 20712 /F
Other Platforms other than windows :
kill -9 20712 or kill 20712