No, once you view a Snap story, there's no official way to 'unview' it. Snapchat doesn't have a feature for this. So be careful when you open someone's story as it can't be undone.
Unfortunately, there's no way to unview someone's Snap story. Snapchat is designed in such a way that once you've seen a story, it's counted as a view, and there's no option to reverse that. It's part of the privacy and functionality design of the app.
Honestly, 'unviewing' a story isn't really possible. Once it's in your mind, it's there. But you could try to forget about it by focusing on other things.