Did you want to tag a really annoying person you know as spam but were worried how that would affect your Google Voice account or the person's ability to call others? Fear not! Your tags as spam stay with you. According to Google: "Tagging a caller as spam only affects your account and is not shared with other users." So, even though Google is "blocking tens of thousands of calls on a daily basis", do not worry if that annoying person will lose their ability to call people because the blocks only affect "automated dialers that call thousands of numbers every day".
What is in it for you to tag someone as spam then? In Google's words, "future calls from that number will not ring your phones, will not be in your messages when you check voicemails from your phone, but will end up in a Spam folder that you can check whenever you feel like it, similar to your Gmail spam filter."
I hope this helps. :) I have been using GrandCentral and Google Voice for a while now, and am currently experimenting with Google Voice as a primary telephone number. If you have been doing the same, or using Google Voice in an innovative way, please share it with the rest of us. :)