They are relentless. i have given up trying to tell them (for the eleventy thousandth time) to STOP CALLING ME. i've not yet figured out how to block the number. But i never answer. And they never leave a message.
I reached out to their Facebook page to complain and got a message back quickly. They told me to go to their contact page (http://www.redcrossblood.org/contact-us) and ask to be removed/have less frequent calls. Fingers crossed that it works...