Given my understanding of how Tor operates (please correct me if I'm wrong, I've never actually used Tor, though the recent government hardon against it has me tempted), it would not be possible to block specific people from using your exit node. Or are you referring to, say, blocking all people coming from that exit node from connecting to some site on the regular Internet? I suppose that should be possible, though justifying it would be hard and the traffic should just route through a different exit node, right?

I'd want to know what makes the FBI think they've identified someone coming through the Tor network. And how.