It's a single line, but you'd need to put it into a shell alias or shell script in order to make it easy to use:
$ kill $(lsof -i tcp:8080 | tail -n +2 | awk '{ print $2 }')
If you want to see and kill processes that don't belong to you, then sudo
needs to get involved:
$ sudo kill $(sudo lsof -i tcp:8080 | tail -n +2 | awk '{ print $2 }')