>spend months working hard on a build
>make it public!
>one person joins, saves, then leaves
>that guy hosts a server with your build which gets popular
>you spent months making something so that someone else could take credit for it.
I agree with Fawkes on this one.
I'd much rather someone else were to take my big build and host it and people were to have fun on it, rather than it sitting in my saves folder waiting to be lost the next time my computer breaks and needs wiping.
If they take credit for it and say so on the Forums then you can easily argue against it.
I personally end up having loads of saves of my builds at different stages of construction, which if I ever wanted to defend something as mine, I could use those as evidence.
But ultimately I'm happy if people are getting something out of my build.
And it saves me having to host it for lots of people to play on, since I am a lazy host.
Also, if it's incomplete then don't worry about it.
Continue building yours. Ban him from your server and continue to build yours up.
He will always have a save that is out of date. You will have a better and continuing project.
And even if he builds his up too, then he won't be able to recreate your style completely. That's pretty hard to do. And even if he can, he won't have the exact same ideas that you have. So you'll both have two very different builds. Yours will likely be better because at least everything fits together and matches.