You can enforce this the same way you'd enforce someone copying a blog post you wrote on a personal blog.
Quora's goal with the "Not for Reproduction" clause is just to give users the right to opt out of the default case and prevent reproduction of their content outside of Quora.
If someone copies your content and somehow isn't able to stop a hosting provider from distributing it (as in the case of a website which they are not an admin) you could send the hosting provider a DMCA (Digital Millenia Copyright Act) takedown notice - this will legally obligate the hosting provider to take the content down. Please note that a DMCA claim only applies when the hosting provider is located within the United States.