Open up the Spotlight Preference Pane in System Preferences, then click the Privacy button and add your card reader or camera to the list there. If you're using a reader, you will probably need a card in it for it to show up in the Finder and allow you to select it in the Spotlight Privacy pane. Since all of my cards have the same name (created by the camera), I think you'll only need to do this with one card (not positive, though).