You want to prevent a specific TCP service from being invoked on your system by inetd .
To disable, comment out the service's line in /etc/inetd.conf by preceding it with a hash mark (#). For example, for the Telnet daemon:
/etc/inetd.conf: # telnet stream tcp nowait root /usr/sbin/in.telnetd in.telnetd
Then inform inetd by signal to pick up your changes. (Here the hash mark is the root shell prompt, not a comment symbol.)
# kill -HUP `pidof inetd`
To enable, uncomment the same line and send SIGHUP again.
Instead of disabling the service, you could delete the line in the inetd configuration file, or even delete its executable from the machine, but such deletions are harder to undo. (Don't remove the executable and leave the service enabled, or inetd will still try to run it, and will complain.) Alternatively, use ipchains or iptables [Recipe 2.6] to keep the service runnable, just not by remote request.