Hey Marci - right now, there's no way to password protect a page.
Our Product Team has chatted about this before but we've prioritized for other projects, such as our our new feature that allows you to download & upload an Unbounce page as a .unbounce file (this was by far one of our top feature requests).
That being said, we're always considering new product features! May I ask, what's your use case? The more we can learn about why this feature is important to you, and for other customers, the more likely we'll prioritize it on our product roadmap. Thanks for sharing!