On the server console, execute:
SHOW USERS DROP
This drops all inactive connections on the server. After you do that and do a regular Show Users, you will see no one or only the currently active users who are actually on the system right now. It should wipe out all the old connections, but if you do it and still see a connection with anything but 0 next to it, it's a bad or phantom session and is indicative of other problems.