Let me assure you that charities would not "waste" their valuable resources unless these mail shots were effective. They bring in money from the general public - including from "repeat asks".
It would be nice to think that all charities had the sophisticated computer systems and high staffing levels required to ensure that when someone asks for fewer mailings than usual, that is precisely what they get. But they haven't.
Can we reach a compromise? When you get three appeals a year from a charity to whom you wish to donate annually - donate annually. Please, though, take a little time to learn more about the work that you are so generously supporting. For with each appeal comes information. Enjoy it! You never know ... the more you learn of the problems alleviated by these charities, the more you may be tempted to give.
Prison Reform Trust