Go to the Red Cross blood donation website. Log in or create an account. Somewhere in your profile there is a spot where you can choose weather or not they are allowed to call. They will stop once that box is checked (or unchecked, I don't remember exactly what the box says.) I had do that a few years ago. Apparently they are so desperate for donors they want you to always have an appointment. My schedule isn't stable enough to do that.
But please donate. You can schedule on line. My next donation is scheduled for October 5. It will be my 128th pint. That's 16 GALLONS. Surely you can spare a pint, too.