I have been dealing with unwanted charity giveaways since I first ordered a magazine.
Always check your contract when you subscribe to a book club, magazine, etc. Even online, there is usually a disclaimer telling you that: a) they vow not to sell your name to a mass mailing list; b) they will give your name to a select few who may meet your interests, if you agree; or c) they make no promises.
As for those charities that will not quit? If they send you a postage-paid envelope, send the order card back marked “Do not want” or in some cases “Deceased” and someone will pay attention.
Don’t feel guilty about using their postage. They will be saving themselves money by not sending their freebies to you in the long run.