On Android 4.4 (Moto X, fairly stock Android), specific contacts have a setting to avoid ringing the phone. I add spammers to a contact called Asshole with that

On Android 4.4 (Moto X, fairly stock Android), specific contacts have a setting to avoid ringing the phone. I add spammers to a contact called Asshole with that setting set. Works nicely, and doesn't require installing an application.