I also suffered from this last year, but had to just accept that it was a fact of life.

It's not just forums that are the problems but search engines which ignore robots text files and list download page. You will never stop the sharing but you can reduce it by using download protectors.

One other thing that i found very useful is that if anyone asks for a review copy convert it to pdf there and then and add a something unique into the pdf. Then when you find your product shared you know who shared it.