I'm not aware of any possible way to "re-direct" visitors who view hotlinked files.
The only possible way is is the hotlinks link like this:
http://domain.com/hotlinkedpic.jpg
users clicking on the above could be redirected, but most pic hotlinkers simply do this:
<img src="http://domain.com/hotlinkedpic.jpg">
there's no way to redirect it when the hotlinked images is embedded in his web page. The most we can o is show a broken image or show an alternative image. which brings me back to how do we prevent this from happening to legitimate users who are behind firewalls or whose ISP's use a proxy.
There's got to be a way, surely!