How do I kill a dead session? Ie, a session where the user is not connected to the ssh-session anymore, but the session still lingers on the ssh-server.
 

From http://systembash.com/content/dead-linux-users/.

  1. Check for the user's session and the pty/n: # w
  2. Enter: ps fax|grep pty/0
  3. Note the process number for the sshd process from §3
  4. Enter: kill <process number>
  5. Repeat §1 to check that the user's been logged off.
  6. Done.