Gotcha. Thanks for the clarification. My guess is that those accounts aren't created by JFBConnect. The easiest way to check is to use the User Map area in JFBConnect and simply look that user up. If they aren't in there, they didn't create their account using Facebook.
Beyond that, I'd recommend checking out a lengthy forum post we made,
How to handle spammy Joomla new user registrations
. That was a response to a similar question and has a lot of suggestions on how to handle spammy registrations.
Check that out and let us know if you have any other questions.
Thanks, and good luck,
Alex