Problem is I can fake a download as easly as I can fake a page view - really all it is just another click that can be automated.

You might be able to implement a system like yahoo or some other sites use to prevent bots - basically it's an image file generated on the fly that requires you to type in security code.

The drawback though is that doing this one or twice is not so cumbersome, but if your downloading quite a few models this is going to get really old, really fast.