Reject call, still leaves voicemail

Hi,
I have a Galaxy S5 and keep receiving calls from unknown numbers which I'd like to block. After adding them to the "reject list", the phone no longer rings, but the user is apparently still able to leave a voicemail. How do I disable this ability as well?

How can I reject a "RESTRICTED" number?

Is any of this possible to do at the carrier level? My carrier is Sprint, and I'd like to reject the number before it even ever reaches my phone.

Thanks!

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Reject call, still leaves voicemail