Clearing backstack in NavigationService

You can use NavigationService.RemoveBackEntry:

 http://msdn.microsoft.com/en-us/library/system.windows.navigation.navigationservice.removebackentry%28v=VS.92%29.aspx

For instance, to remove all entries from the stack:

while (this.NavigationService.BackStack.Any())
{
   this.NavigationService.RemoveBackEntry();
}

Also, if you want to remove only the previous page after checking its URI:

var previousPage = this.NavigationService.BackStack.FirstOrDefault();

if (previousPage != null && previousPage.Source.ToString().StartsWith("/MainPage.xaml"))
{
    this.NavigationService.RemoveBackEntry();
}