By default Access server verifies username, password, and certificate, for VPN connections. If you use autologin profiles, only the certificate is sufficient. But without certificate you cannot connect in any case.
If you want to prevent people from getting the certificate from the access server web interface, you can switch on the option to only allow admin users to access the web interface. But then you will have to take care of distributing the client profiles by hand, for example via the command line options.
https://openvpn.net/vpn-server-resources/create-connection-profiles-and-connect-client-installers/