How do I force sshd to only accept protocol version 2?

The Protocol directive in sshd_config is used to set allowed protocols.

Backup the current configuration and edit it with your favorite editor:

root@root [~]# cd /etc/ssh/

root@root [/etc/ssh]# cp sshd_config sshd_config.save

root@root [/etc/ssh]# vi sshd_config

root@root [/etc/ssh]# diff sshd_config sshd_config.save

14c14

< Protocol 2

—-

> #Protocol 2,1

root@root [/etc/ssh]# /etc/init.d/sshd restart

Stopping
sshd:
[ OK ]

Starting
sshd:
[ OK ]

root@root [/etc/ssh]#

Sshd should no longer allow protocol 1 authentication:

root@root [/etc/ssh]# ssh -1 127.0.0.1

Protocol major versions differ: 1 vs. 2