If you have a console or RLM/BMC connection you can do "logout telnet" and this will disconnect any SSH sessions.
If you have a already trusted configured SSH or RSH client (SSH in terms of issuing SSH commands and scripting, not in terms of full SSH sessions), then you can do the same thing like "rsh filer_name logout telnet" or "ssh filer_name logout telnet".
Without any of the above, you'll just have to wait for it to timeout I'm afraid. Default is 60 minutes, but I'd recommend dropping this to 5 or 10 minutes for SSH.