I think ASPCA sold my name/address to similar organizations-how do i put an end to it?

Last december Aspca - american society for the prevention of cruelty to animals- well they were have a donation drive or something where they wanted to meet a certain target before before the year ended. I had read up about them and was interested in making donations.
So i donated $ 50 i think i don't remember, through credit card- once. and since then not only have i been contacted by them through mail asking for a donation with a story about a kitty cat with a broken leg and all they do to help animals but they need my help too blah blah. and occasionally a free gift like address labels or magnets.
I'd say i've gotten 30 letters from them alone in a 7 month period. At the same time i was also getting mail from organizations i have never contacted ever before all similar -
worldwildlife.org, the humane society, defenders of wildlife, north shore animal legue. most coming with free gifts and asking for donation.
So that lets me to think that if they are giving away free stuff these organizations- they must be connected to ASPCA some how- which makes me suspicious. I have yet not replied to anyone of them.

How do i make this end? Do they plan to guilt me with the free stuff if the sob stories don't do it.
I'm disapointed- i do one charitable thing and my mailbox is crammed with mail.

Same thing happened to me... They make additional money by selling your mailing address to other organizations... I can't afford, nor do I want to, make donations to all of these organizations... However, I'm not as upset about their mailings as you seem to be... I just throw their ads away... but I suppose you could use their "postage paid" envelopes to request they take you off their mailing lists.

P.S. The free gifts are just that... "free"... some I save some for donations to my local humane societies charity auctions.