In most implementations a client can close his own sessions. This is especially helpful when dealing with problematic transactions or RFCs that hang and do not release the session.
GO to SM04 then user list will appear, Double-clicking on the User will bring up the sessions he has open. To close a session, select it and then click the "End Session" button.
Sometimes there will be a requirement to terminate a user session.
For example: An user has run a report or program with inappropriate selection criteria, which leads work process going to PRIV mode occupying so much memory impacting performance of the system. In those cases, you will have to check with the user and terminate his session or logoff user system wide if he is no longer working.