It's already been stated that you can't block calls based on the Caller ID Name (CNAM). But, understand that the CNAM field is irrelevant with regard to call blocking. Given that Google Voice doesn't use carrier-based CNAM at all, whatever characters are in the name field of a linked phone number's caller ID display are not provided by Google Voice or its carrier partners.
The phrase "UNKNOWN" or "ANONYMOUS" or "PRIVATE CALLER" or some other wording are substituted by the carrier receiving the forwarded call (e.g. your mobile or land line phone company), or are added by your telephone itself.
The calls you actually would want to block are calls that contain no caller ID
number. Google Voice itself can usually recognize these calls and send them directly to your voicemail greeting, if you configure it to do so, via the old, "Legacy" website. This happens before the call would otherwise be sent to your devices or numbers. Spammers/robocallers rarely leave voicemail, since their predictive dialers typically ignore calls not answered by a live person.
Sign into the correct Google/Gmail account that holds your Google Voice number. Go here:
https://www.google.com/voice#groupsEdit the "Anonymous callers" row, and disable (un-check) all of your linked devices and numbers. Save the setting. See the screenshot below.