Are you worried about snail mail spam or just email spam? The latter is easy to deal with. I have a dedicated email address which is the one I give out to computers that ask for one, to take the spam away from my others. Also, what charities are you afraid of spam from? In the year I've been donating to MIRI, I've gotten only one random email from them. Givewell also has this, which will pass on your money in their recommended split among their top charities. Seeing as they seem to be all about donating to a charity because it is the best and you want to do good, and not because you have received guilt-inducing spam directing your donations to a particular charity, I would be surprised if they spammed their donors. Perhaps someone who donates to them can confirm.