you call them and tell them to put you on their donotcall list.

if they don't - you report them to the DONOTCALL.GOV website and file a complaint -
Click on the File a Complaint button
You have to report them in order to stop them - otherwise they do not KNOW

Cancer Support Services @1-888-276-2976 is pestering me.  I am on the "Do not call" list.  What else can one do?