No. Anyone can download the HTML from an online version and just import that into Twine.
That said, I've seen tons of Twine games, and I've never heard of anyone "stealing" a game like that. (Really, it's just copying, you're still the one who knows best how it all works and such.)
I wouldn't worry about it that much. The same is true to one degree or another for just about any page you see on the web.