There’s no way to stop it, which (imo) is what makes the internet great. Regardless of what the law says, anything on the internet is basically public domain. If you manage to shut one guy down, ten more will pop up in his place.
If your client intends to charge for these photographs, then I assume they’re either somewhat scandalous, particularly embarassing or oddly enlightening—in which case he shouldn’t have a problem finding a legitimate publisher to take them off his hands.
Celebrity photographs typically have value only when nobody else has seen them. Think about how fast the value of a car drops once you drive it off the lot. Multiply that by 1,000, and you’ll have an idea of how fast the value of a photo drops once it’s in the public domain of the internet, or once it’s printed in a magazine.
If a legitimate publisher isn’t willing to pay for the pics, then chances are they’re probably overvalued by the client IMO. Unless it’s porn, that is.