I don't think comcast does that
You do realize I used to do this for a year straight. Then I just gave up and started saying stuff like "Kill yourself for being a nuisance to the world", "Stop loving calling me" etc etc. They never really remove you.
Comcast digital voice does. I do it all the time.http://web.mail.comcast.net/zimbra/mail?app=voiceMissed calls > Right click the number > Call screening
inb4ohtheinrony