One day I got so sick of getting "You're Pre-Approved!" junk mail that I decided to see if I could do something about it. I got on a roll and found ways to stop all sorts of junk that I get. Phone calls, mail, etc. Not email though. You can't stop junk email, but paper mail is another story :-)
Also, remember that if you don't want to give any of these web sites your real email address, you can use a temporary email address from http://www.ipsite.org/temp_email