Make a new plan then Stan and try blocking the number, or turn off your answering machine and let the phone ring. Turning off your ringer might work, too.
Block call if you will but I say in addition to that everyone receiving such calls should call the number back 100 times one after the other with the first 99 being hang ups as soon as the call is answered. On the 100th, who ever answers should be told that there will be 100 calls every single day for 100 days that consist of 99 hang ups and a reminder on how many days of calls remains. Disrupt their business enough and their business model will change.