How can I prevent a freelancer from stealing my idea?

Technically, you can never prevent someone from stealing anything, you can simply have documentation in place to file suit to attempt to recover damages in the event of theft. An NDA is typically the first document in that chain of protection, and an important step in the development stages. It is also important to retain all of your notes, documents and correspondence. An NDA will not prevent theft and would likely be difficult to enforce in Denmark without legal counsel there. The next step in the legal documentation process and your option at this point would be to seek IP (intellectual Property ) protection for the idea. If your concept has sufficient "originality" you may be able to seek patent protection. You should file at least a provisional application fairly soon especially of you believe there may be some potential for the designer to try to claim the idea as his. A patent can be enforced in the United States even if the infringer is in another country.