Prevent or cancel navigation?

[gleclair]

First off, many thanks to @pomianowski and other contributors for creating and maintaining this solution, your work is very much appreciated!!
My question is: Is there a way for a view model to disable or cancel navigation? If a user is in the middle of editing, I want them to either save or discard changes before moving to another screen. Maybe I could disable the navigation menu when editing starts, or wire into an event to cancel it? Hopefully I'm missing something easy, any suggestions are appreciated.