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:

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