• Create a new revocation list from System → CertManager → CertificateRevocation.
• Add the certificates that you do not want to be active any more.
• Assign the new revocation list to the vpn server in my case VPN → OpenVPN → Servers.

You can easily choose your revocation list from the Peer Certificate Revocation list.

Navigate to System → Cert Manager.

Select Certificate Revocation.

• Click Add or Import CRL.

In Create new Revocation List:

• Method: Create an Internal Certificate Revocation List..
• Descriptive name: ShareWiz OpenVPN - Revocation List.
• Certificate Authority: ShareWiz OpenVPN - CA. Select here a CA that is already created.

In Internal Certificate Revocation List:

• Lifetime (Days): 3650.
• Serial: 0. Default.

• Click Save.