If you need to disable the telnet function for a particular UNIX/Linux user, it can be done by editing the /etc/passwd file and replace the shell (say /bin/bash) for that user’s entry with something else (say say /dev/null).
If it doesn’t work, then do these:-
- Login as the user.
- Edit the .profile (with vi editor) and add in the last line “exit”.
- Save the .profile.
With that, if the user tries to do a telnet, he will be automatically kicked out.