Before using below function, you need to include Microsoft.Phone.Controls nampespace in your project.
using Microsoft.Phone.Controls;

public static void RemoveAllNavigationBack()
{
if (!((PhoneApplicationFrame)Application.Current.RootVisual).BackStack.Any()) return;
var length = ((PhoneApplicationFrame)Application.Current.RootVisual).BackStack.Count();
var i = 0;
while (i < length)
{
((PhoneApplicationFrame)Application.Current.RootVisual).RemoveBackEntry();
i++;
}
}

Or, you can remove Navigation Backstack by your choice.
public static void RemoveNavigationBack(int count)
{
for (var i = 0; i < count; i++)
{
((PhoneApplicationFrame)Application.Current.RootVisual).RemoveBackEntry();
}
}

I hope you find this post helpful, Thanks for reading.