Google has a Global Spam Filtering system that would automatically redirect calls, texts, and voicemails from any of the numbers in the Google database that have been identified as spam directly into your spam folder. But you will need to enable this feature for it to work.
To enable this feature:
1. Log on to your account at http://voice.google.com
2. Click the wrench icon and select Voice Settings.
3. Click the Calls tab.
4. Check the box next to Global Spam Filtering. at the bottom of the page. See screenshot below.
If you’ve marked a call or voicemail as spam on the Google Voice website, any future calls from the caller will be sent to your voicemail, and your phones won’t ring. The voicemail will be automatically placed in the Spam folder. Just find the call or voicemail from the caller, select the checkbox next to the call or voicemail, and click the Spam button.
You can also mark a caller as spam when listening to your voicemails by pressing 8 at any time during the message.