The way I would look at it, if they've already 
taken your pictures anyway (before asking) then 
they might as well have not asked. I would 
immediately put watermarks on everything on your 
page (to prevent further problems) and demand all 
of your images be removed from the other person's 
page. No you can't *make* them do it, but if you 
have anything you've edited heavily enough to 
make it more "your own", you could easily 
complain to their ISP (the one supporting the 
site) and that might create just enough trouble 
for them that they will comply.

But watermarking is the only way to assure people 
won't bother to steal your scans. Trust me. . . 
Before I picked up Psychic Force Puzzle Taisen, I 
found someone who had watermarked scans of the 
album pictures from the game. . . I tried damn 
near everything to remove those watermarks and 
nothing would do it since the pictures had been 
saved as JPGs. (Don't save as a 'grouped' image.) 
The only way I got rid of them was to spend over 
three hours per picture removing the marks by 
hand. Most people won't bother!

And incidentally, I wouldn't recommend allowing 
anyone to take your images for use on a public 
site no matter what the stipulations. Once 
they're off your site, it won't be YOU people ask 
if they want to use the image. The second person 
can easily hand it out to half the world.

Many Sharp Smiles,
--Drac (below is how I feel about public use of
        stolen scans. . . "Steal my scan, will
        you?!?!?" ^_~)