You just need a smarter phone. There's some Gigaset models that will do custom ringtones based on caller ID.
Last time I had a landline I only ever used it for outbound calls. I kept the ringer turned off. I connected an answering machine to it and set it to "message" mode. Recorded a very long message with lots of "hmm", "tell me more", and "can you repeat that, you broke up" just to screw with the spammers