How to stop a VPN server launched with vpnd
Two Ways to Kill a Process (sung to the tune of "Fifty Ways to Leave Your Lover")

First Way:


  
  Launch Activity Monitor 
  Locate the daemon/process you want to halt (vpnd in this case), and select it (white letters on blue field)  
  Click the x button in the ULHC, Force a process to quit
  Confirm by clicking Quit, or Force Quit if it's stubborn :) 
  


Second Way:
If you're keen on the command line, why not use kill? see man kill


  
  Get the PID for your daemon vpnd: ps -ax | grep vpnd, 
  kill PID (where PID of course is the actual PID number)