Let's think of this in a larger context: why do you want your content that you worked so hard to produce not to be shared with others? Doesn't it benefit you in the long run to have that content shared between colleagues than gating it and turning them away?
I just see this question all the time, and I think it's something you don't really need to worry about. If someone forwards your whitepaper...great news for you, and you have the original person's name.
In your case, just put the thank you page link in an email if you are that worried.
As marketers we need to see the forest for the trees sometimes. 🙂
</Monday Rant>