As somebody mentioned earlier, you can stop the pre-approved credit card offers by

1. Call 1-888-5-OPTOUT.
2. Select option 2 to skip a message about an internet email (described below).
3. Select option 2 to opt out of mailings permanently. Be careful, because option 1 only opts you out for two years.
4. Follow the prompts to enter your phone number, name, zip code, address, and social security number.

The service states that your request will be processed within 5 business days, and a "Notice of Election" form will be mailed to you.

They also have a web site equivalent to do online. Can't find it right now.

EDIT: found it https://www.optoutprescreen.com/ Make sure you select permanent opt-out.