By default rails does not clear out stale sessions from the session store. To implement this feature I added the following small snippet of code;

class SessionCleaner
def self.remove\_stale\_sessions
CGI::Session::ActiveRecordStore::Session.
destroy\_all( \[‘updated\_on <?’, 20.minutes.ago\] )
end
end

And then invoke the remove_stale_sessions method every 10 minutes via;

*/10 * * * * ruby /full/path/to/script/runner 
   -e production "SessionCleaner.remove_stale_sessions"