I think the celebrities should sue these people. Allow me to explain:
If you, dear reader, were in business, you could not use Britney Spears image in order to promote your business ("Britney Loves My Wazoos! You Will Too!"). Not only could she sue you and make you stop using her image, you would probably have to pay her court costs and possibly some type of punative damages.
Now, these 'professional' celebrity photographers: they are in business. Their business is taking photographs of celebrities and selling said photographic images to magazines. In other words, they take the photographs for profit.
It has long been settled that celebrities can prevent other people or businesses from profiting from the celebrities image in the pursuit of business without first obtaining the celebrities permission, license, etc. Why, then, should the paparazzi (as well as the magazines) be exempt?
I submit that, at the very least, there should be a law that if a paparazzi takes an unauthorized photograph of a celebrity, and sells said photograph, then the celebrity automatically receives a percentage of the sale price. Furthermore, the magazine that publishes the photographs should also have to automatically pay a price.
Anyway, hurt them in the pocketbook, and the practice will certainly slow down.