You can `readonly TMOUT=300; export TMOUT` to have bash (for CSH/TCSH try `set -r autologout= 5`) to you off automatically if you leave the shell idle for more than 5 minutes. The variable is read-only, so users can not change the value. See www.cyberciti.biz/faq/linux-un... for more info.

Linux / UNIX Automatically Logout BASH / TCSH / SSH Users After a Period of Inactivity

Explains how to automatically log users out after a period of inactivity under Linux / UNIX / BSD server and ssh clients to improve security.