David,
You'll need to make a quick modification to JFBConnect to do this. Please edit the /components/com_jfbconnect/controllers/loginregister.php file. Do a search for "sendNewUserEmails". It will come up twice. The first time, the line will look like:
$this->sendNewUserEmails($jUser);
Simply comment that out by adding two foward-slashes, like so:
// $this->sendNewUserEmails($jUser);
That should prevent the emails from being sent. Obviously test, and let us know how it goes!
Thanks,
Alex