Is there fuser -k  analogue for Mac OS?
Save this in some file that is in your PATH, call it kill-server for example. Make sure to chmod +x on it.

#! /bin/bash

lsof -i tcp:8000 | grep -v PID | awk '{print $2}' | xargs kill


Then invoke it with kill-server or whatever you decided to call it. This is about as short as it gets.