The only way I know of is through something like YouMail, and it doesn't stop them from calling, it just answers with a message to unblock the number and call back, and hangs up so they can't leave a voicemail if their number is blocked.
Since you can only block specific numbers, and a blocked number is unavailable/unreadable, it's difficult to stop the call from reaching your phone. Once it reaches your phone, you can deal with it through YouMail or another voicemail app.
I feel you! Calls from private numbers can be very annoying. We offer a solution that blocks private and restricted numbers! You simply have to add Usage Controls to your line and select to block calls from such numbers. Get all details at this link http://vz.to/plz3pA and review number 6 at the same link for instructions to add Usage Controls to your account.
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