permitting only S/Key login for telnet

[Christian Lecompte <cl@fidsec.com>]

I'm playing around with S/Key and would like to limit telnet logins to 
S/Key only.

Here's how I do it with sshd:
PasswordAuthentication no
ChallengeResponseAuthentication yes

But I also would like this type of setup for telnet.

Is there a way to do this without modifying the login source for telnet?