Kill a process using a specific port

First, find which PID is using the given port (replace 8080 with your own value):

lsof -t -i:8080

This will give you a PID, like 12345.

Next, kill the process by id (again, replace the id with the result of the previous command):

sudo kill -9 12345

You can also do it all in one command:

sudo kill -9 $(sudo lsof -t -i:8080)

If for whatever reason something is not working, you can get more info about what processes are running using ps ax