How about buying the VERY PRETTY BEAUTIFUL text editor (That is the program name), when you buy the game. So when BeamNG is released, along with the game and cars and maps, the text editor install is also included, for free, with the $20-25 asking price for Beam?
What about an svn repository, concealed as a button on the forums, which is available to forum members who either log in, or have a key code? If your wondering on what an svn repository is, it's basically a special kind of file type which can be updated often, all through one link. Files to download an svn are free, so it's easy peasy. I don't really know how to upload stuff up on an svn, but surely it isn't to hard.
There is no way to stop it 100% Never ever will be possible to do so. I can only think of compiling Lua(aww yeah...) into Lua-Bytecode(like Java has) or something like that, but it would still be possible to read, if you just find out what is what. Or some sort of encryption, which can be broken either by getting it from the game exe or the encryptor exe, so it doesnt work aswell. The other thing is having something such as the Steam workshop, you wouldn't have to protect your files because your can only access them through the steam workshop in the game, and if someone uploads your creation, you can report it and it's taken down. You would get the addon and you can edit it and do what you want, but they will all be in one place, meaning that there is no point in going to other sites or w/e.
Or encode the files on a quantum hard drive using qubits, so that if someone tries to read the information superimposed on it, it collapses into a single waveform and becomes unreadable. problem is, you can only play the game through indirect observation but its more secure
