If you have a NAS licensed, you could use a timer trigger to check the Session table (2000000009) for users which exceed a certain “Idle Time”.
But you need to be careful to only kill those users which run on the same Database as the one listed in “My Session” and that you kill users which have “Microsoft Business Solutions-Navision client” in the “Application Name” field only.
Just set a filter according to the rules stated above and call a Delete on the session table. This will disconnect the user.