I'm in the same position but instead of copying pages, people are redistributing files that I made. They do end up leaving a link back to the website but they're taking away income that I could be generating through download pages.
I think you shouldn't take this too lightly. You're only receiving a small number of visitors compared to what you could be receiving if they only posted a paragraph and told users to go to your page to read the entire article. Contact the forum's administrator and ask them to remove everything except a short paragraph. They'll probably understand.