Hi,
There are two easy commands for that:
/etc/init.d/openvpn stop (or disable if you wish for it to stay disabled).
/etc/init.d/openvpn start (to start it back up).
Also, if you go to /etc/init.d/ directory you will find a lot more services, not only OpenVPN, so you can use the same commands to stop and start other services too.
EB.