1 - Don't put them on Flickr!
but as you did and they nicked them (as can be done with pics on most sites, including this one!)
2 - Send them a big invoice. As they took them without permission and didn't negotiate a fee, set your usual fee as a starting point for negotiations (£1,000 should do, a lot but not out of the realms of possibility), give them 30 days to pay and if they don't then tell them you're pursuing them through the small claims court.
3 - If you don't already, embed a copyright notice in the EXIF data of the images as they're taken, you should be able to find out how to do this from the camera manual, it usually involves connecting the camera to your PC with the camera software on and going into the tools menu.
On a similar note, I had an email from
www.kentattractions.co.uk yesterday who have a connection with visitkent. They have a photo competition going for the best pics of their members attractions, but in the terms and conditions they have -
"By entering the competition, the Association of Tourist Attractions in Kent (ATAK) obtains full copyright of the submitted entries. All entrants grant the Association of Tourist Attractions in Kent (ATAK) a worldwide royalty free perpetual licence to publish and use entries in any media (including online) for publicity and news purposes"
Most of their members have provided the prizes, so for free they get an archive of photos of Kent to use themselves and sell around the world. A nice scam and I had pleasure in telling them so!