I think the best way to approach this is to start off with the assumption that it's a mistake. You'd be surprised how many people think posting a photo on flickr automatically places it in the public domain, even people who should know better.

First, politely request that the photo be (a) credited or (b) taken down. If it has been used in a commercial sense, you could also include an invoice for an amount you feel comfortable charging.

If that doesn't work, then try a firmer reply, including an extract from the part of the Flickr terms and conditions that have been breached, and mention that you will be taking legal advice if they do not comply.

At this point, if you've still had no joy, you could do one of two things: contact a lawyer and ask them to send a letter on your behalf, or (and this is cheaper) write about the breach in as many public forums as possible, e.g. on Twitter and Facebook, where there are plenty of people who will express outrage on your behalf and pass your message on. That might be enough for them to get the message.

Failing that, you would probably have to go to the small claims court, to claim for non-payment of your invoice.