I received this weekend a preapproved offer to apply for the Discover card. Off and on for the past 10+ years, they've been trying to get me as a cardholder. When they finally called me, I told them to go away and take me off any future offers.
Because another offer got sent after I told them to stop, this is what I do to all credit card companies when they don't take no for an answer. Whenever I get an offer in the mail, I take the application that requires a signature and tear it up. Not in shreds, but just enough so that my name can be still be seen. I then take the shredded form, place in the postage paid envelope so graciously provided, and mail it back to them. Sometimes it usually takes 2-3 mailings from the same card issuer, but eventually it stops.
Of course, what will get phone calls and mailings to stop is to call the various agencies to "opt out". A few, such as the Discover mailing, will still sneak through the cracks. Why card issuers think these forms of solicitation still work is beyond me.