====== Ubuntu - SSH - Blacklist a public key ======

You could remove the keys from each respective **.ssh/authorized_keys** file, but to do it on a server-wide basis.

Look into the **RevokedKeys** configuration option for the sshd.  See man sshd_config.

Remove john@server1.sharewiz.net from ~/.ssh/authorized_keys and lock the user account too:

<code bash>
cd /home/john
sed -i '/ john@server1.sharewiz.net$/d' ~/.ssh/authorized_keys
passwd -l john
</code>