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