Hello viswa,

1. Presuming that you would like to disable the navigation from one root element to another when you are currently focusing the submenu of the first root element, I can suggest you handle the PreviewKeyDown event of the XamMenu and set the e.Handled property to true if the Left or Right key has been pressed. This will not allow for the default navigation logic to be executed and we will remain within the same root element.

if (e.Key == Key.Left || e.Key == Key.Right)
    e.Handled = true;

2. In order to disable the cyclic navigation for the XamMenu (being able to select the first item when pressing the right arrow key on the last and vice versa), you can handle the PreviewKeyDown event as well by adding an explicit check if the first or last item is currently with an open submenu.

var rootMenuItems = FindVisualChildren<XamMenuItem>(menu).Where(i => i.TemplatedParent == null)
                                                            .OfType<XamMenuItem>()
                                                            .ToList();

if ((rootMenuItems.FirstOrDefault().IsSubmenuOpen && e.Key == Key.Left) ||
    (rootMenuItems.LastOrDefault().IsSubmenuOpen && e.Key == Key.Right))
    e.Handled = true;

I have attached a sample application that demonstrates the approaches from above.

If you have any questions, please let me know.