I was surprised a few days ago to find one of my flickr images along with its description featuring prominently on a page at a website that linked locations to flickr images for tourists called wiki.worldflicks.org. It linked back to my flickr site, so they would claim that it improves my flickr traffic, but the image is clearly marked "All Rights Reserved," and they certainly didn't ask. Worldflicks looks like it makes money by serving google ads with their pages that include a map of the location and featured and linked images and videos. Had they asked, as other website owners have, I would have offered it at the rates that Alamy charges for similar images and usage, since I don't think the "free exposure" is that valuable in this case.
So I used whois to look up the site owner and gave them the opportunity to license the image for a modest fee or to remove the link, which they did promptly, since they link to thousands of images via an automated script that uses geotagging information on flickr and can't afford to pay for them. Because they are doing this by script, their pages aren't too reliable. Now that my image has been removed, for instance, the top image of the location does not point to the moderately well known building that is the subject of the page, but to an interior hallway of a building across the street.
Looking into this further, it seems that permission for this kind of unauthorized usage is turned on on flickr by default. I don't geotag any of my images, but it seems that flickr may have a script that does this based on keywords. You can batch edit your flickr stream for geotagging, so I identified the three tagged images in my flickr stream and removed the geotagging.
To turn off the default permission for third-party usage go to You>Your Account>Privacy & Permissions and click on "edit" under "Hide your stuff from public searches" and check "Hide your photos from searches on 3rd party sites that use the API, including Yahoo! Image Search?" This doesn't seem to hide images from google, which I think is good, since I don't mind traffic coming to my site via google searches, but it should block usage by sites like worldflicks.
You may also want to scroll down the Privacy & Permissions page to "Who will be able to see your stuff on a map" to restrict that as well, which I did, though I'd think that removing geotagging should take care of that.