Comment 46 for bug 49594

Here's my workaround that, once set up, requires no further intervention by the sysadmin (me) in our corporate environment (see https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/236210/comments/6):

In short, we are creating a shell script to kill all the remaining user processes after logging out of GNOME, setting that script suid root, and having gdm call that script upon log out.

$ cat /usr/local/bin/kill-stragglers-auto
#!/bin/bash

# takes 1 argument: the username of the user who logged off

USER=$1
sudo kill -9 $( ps U $USER | grep -v TTY | awk '{print $1}' ) 2> /tmp/kill-stragglers-auto.errors.log
sudo /etc/init.d/gdm restart

$ sudo chmod +s /usr/local/bin/kill-stragglers-auto

$ ls -lh /usr/local/bin/kill-stragglers-auto
-rwsr-sr-x 1 root root 213 Aug 27 18:32 /usr/local/bin/kill-stragglers-auto

$ cd /etc/gdm/PostSession

$ ls -lh
total 8.0K
-rwxr-xr-x 1 root root 395 Aug 27 18:30 Default
-rwxr-xr-x 1 root root 305 Aug 27 18:29 Default.080827.orig

$ tail Default
    fi
  done
  IFS=$OLD_IFS
  echo "$OUTPUT"
}

# the following was added by toobuntu on 080827:
/usr/local/bin/kill-stragglers-auto ${USER}

exit 0

Hope this helps someone!