Every time AARP sends me something (and they do, often) I cram all their literature into the remittance envelope provided with a note saying “I DO NOT WANT TO BE PART OF YOUR PRO-LIBERAL ORGANIZATION!” and send it back to them using their pre-paid postage. It doesn’t stop the mailings but it costs them money and frankly, I enjoy doing it.
See if you can tape it on to a brick or add a some metal to the envelope and let me know the results!