How do I block certificates?

You need to explain what you mean about blocking certificates. Under normal use certificates are invisible, unless you are required to accept a certificate for a specialized application. If this happens you will see a dialog asking if you want to accept the certificate. So if you want to block certificates then just don’t accept them. If you have accepted a certificate and want to change your mind go to Settings/General/VPN & Device Management, tap on the certificate profile, and remove it.