How do I clear the Navigation stack?

In the latest version of Xamarin.Forms you can see your navigation stack using

Navigation.NavigationStack

therefore you could use a

var existingPages = Navigation.NavigationStack.ToList();
foreach(var page in existingPages)
{
    Navigation.RemovePage(page);
}

This code would have to go into your code behind of a Navigation Page or something that implements INavigation.

More information Xamarin.Forms.INavigation Members

This is a function I made to empty the stack and navigate to a specified page. (The use case was the app was de-activated during use and I need to kick the user out)

    public async Task PopAllTo(ViewModel vm)
    {
        if (vm == null) return;
        Page page = PreparePage(vm); //replace 'page' with the page you want to reset to
        if (page == null) return;
        _navigation.InsertPageBefore(page, _navigation.NavigationStack.First());
        await _navigation.PopToRootAsync();
    }