A password reset might not disconnect someone who's already authenticated, so you could try disconnecting all current sessions.
Login as Admin:
Manage -> User Activity -> Online Users (end web session)
Manage -> Current Connections -> SMTP & ActiveSync
For current connections, you could disconnect all or find the IP by looking at the messages in spool. Unfortunately, it doesn't show the username.
But that would be a nice feature request -- show authenticated username in Current Connections.