Just follow the 3 steps below.
1. Get a free google voice number. Go to https://www.google.com/voice and follow the prompt to set it up with your home or cell (you should remove it later) and google chat as the forwarding numbers as shown below ...
Phones setting: http://selvans.net/vonage_scb/gvsettings.png
Calls setting: http://selvans.net/vonage_scb/gvcallsettings.png
Note: mine shows only google chat since I removed all forwarding numbers.
2. Login to your vonage account and setup simulring to ring your google voice as shown below
3. Login to your vonage account and setup voicemail timeout settings as shown below
Vonage VM timeout: http://selvans.net/vonage_scb/vmtimeout.png
After this, when ever you get an unwanted call, login to google voice, select history, find the unwanted number and select "block" from the pulldown menu under "more" as shown below ...
Note: Google does a pretty good job on its own blocking spam... as you can see, I did not have to block this 702-815-2394 number since google already did that for me :)
In addition, if you have a list of numbers to be blocked you can follow my original post below to setup a group of numbers to block.