Probably the easiest one is to do a "/sbin/init.d/inetd stop" and then when you are done do a "/sbin/init.d/inetd start".
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There are several ways to do this.
There is one caveat, "Don't log out yourself".
1) you can kill the inetd daemon. Can't login without a telnet session or FTP session. As above...
2) Move the password file. Can't login without a password.
3) Change permissions on the / directory to 700. This will disallow users from logging in as well. I did this once by accident. You can search for others who have done the same by accident!
4) Reboot into single user mode. Removes all network connectivity.
I am sure there are several other ways, but these are the most useful.
Of course you must either kick out the active users, or ask them to logout for maintenance. Try to be as nice as possible.