There's a 'Blacklisted Numbers' section on the 3CX server.
Though, our recommendation is only to add to this is if you are certain they can be blocked - e.g. it's a known scam or nuisance number. There's a few services to indicate if something is a known issue, Google search "check scam phone numbers uk".
You want to avoid blocking a number that may well also be an outgoing number for something legitimate.
You can let us know the number to block via a support ticket.