That's pretty sad. Sending the names through the offender's list sounds like a great idea to me. On the devil's advocate hand, though, I must say, is it fair to lock-out a registered offender who might or might not be attending with malicious intent?
Though, this does make me question some things about anime conventions. I've heard rumors that some people go to the conventions purely for sex, contra-ban, alcohol and the like. I have doubts that was what within this girl's mind when she attended the convention. And to continue relations with someone clearly older than her online is fine to a degree. But to then meet in person without the protection of parents, public, or the convention atmosphere... :/ Well, yeah. While the minor could say it was consent, by age alone (defining brain development), I would seriously question her motives.
REGARDLESS. I'm totally on the side of the victim and everyone being careful and aware of who they talk to. If you're talking to someone who is invading your personal bubble or making you uncomfortable, you do have the legitamate right to verbally turn them down. And if you don't feel safe doing that, you look for help from someone bigger than you (physically or by age), someone more outspoken than you, or from staff and security. And from that you're probably within in your right to call the police with the excuse of "harassment". [If and only if, the person has not let you alone after repeated requests that they do so.]
^ I think that sounds about right?