One idea is to send them to a page targetted to the niche of the page they were snooping in.
ie: if it's a lesbian free site and they're snooping for pic021.jpg for example, load up a 401 or 404 page with a title like "My secret Lesbian stash.." or something like they feel they've stumbled onto something good. Then present some good no BS marketing to some "favorite" members sites or whatever.
A neat scripting idea would be to have the error page actually pull a pic from the set they were just viewing, and match it to a pay site that has "tons more like this..".
Just imagine where they are, where you know they've been, and give them basically what they want.. provided they pay for it this time
hehe