I have the same problem at one of my client's offices.  I merely place the phone on speaker, then hit whatever number they want me to hit in case I am interested, and then I pound on them, advising them if they call one more time, I will turn this matter over to our Summary Court and sue them.  IT WORKS.

The same goes for those unwanted faxes.  In fact, I went so far as to fax a bill (toll free fax) to them for my time, toner, copy paper, etc.  That worked too!!!

Donna