When they call you just let them know that you will speak to them by email or written letter only and that you are giving them notice that they are not to call you. If need be, let them know that you are recording the conversation. (especially if you lie in a state that requires such notification) They will get the hint.
Also, I remember when I call DRU, there was a machine voice saying something like "the call is monitored/recorded", so I guess I could do recording without telling them since they were supposed to do recording as well. ;-P