I own a phone with an address book that supports multiple phone numbers in a single addressbook entry( I guess most phone's support that now ?). I save the phone-numbers from which I receive 'spammy' phonecalls under the addressbook entry "SPAM". So I am normally able to disconnect most 'spammy' phone calls. Once in a while a new telecaller appears whose number promptly gets stored under the same addressbook entry.

Update: bankaro.com has this idea of providing screened numbers through your trusted network of friends. If you have tried it please let me know how did you find it.