Do they still make Telezappers? Used to have one of those gadgets a long time ago. It would emit the triple-tones to tell the robo-caller that it reached a disconnected number, and that would usually drop it from the database. Granted, it was a pain in the [***] to hear those tones every time you picked up the phone, but it sure beats the alternative! The downside is that you could only use it with a landline, and with more people making their cell phones primary or even dropping landlines altogether, the Telezappers probably don't sell like they used to, if they're even being manufactured at all.
I just try sending a fax to their land line....OVER AND OVER AND OVER until I get bored so that it screams in their ear when they answer.
So they are as annoyed as I am with their calls :)