Add the following line to your servers conf:
Use the easyrsa 'revoke-full ' command to revoke the key.
Copy (NOT MOVE!) the crl.pem file from directory 'keys' to /etc/openvpn
Restart the server
Notice I: The next revocations will be added to the crl.pem file. There is no need to restart the server for further revocations, the server will read the crl.pem file on the fly.
Notice II: Do not worry: The error 23 indicates that the key was revoken and all is fine.