One prepaid envelope at a time.
I geti numerous offers from Citicorp/bank/card every week, and they all come with a prepaid postage envelope. I take out everything with my name on it, and put everything else back into it. Then I stuff all the other "pre-approved" credit card offers in it, again with out my name, and I fill it up when I pretty much can't put anything else in it, I add a piece of paper that says "I support the Second Amendment, not Citicorp", seal it, and mail it.
Imagine 1 million gun owners doing that every week, with every Citicorp offer that arrives.
Pass it on.