I get a lot of them too. It may seem childish, but I take the application, rip it up and put it in the postage paid envelope and send it back. Occasionally I will stick a few paper clips in as well - but only when I am having a really bad day.
Don't send them their app - it IDs you and paperclips are useful. Just send them 20 or thirty pennies from your change tray. It'll cost them more than it costs you. Tape it up real well. NOW and PETA regularly get my donations. ;-)
I do the same thing, but I also include all the little ads that come in the mail in an envelope once a week from ValPak. Occasionally I will fill out the application with bogus name and put the address for another credit card company on the form. I like to think that one credit card company is sending rejection letters to another credit card company.