public static void CloseLayoutPanes( 
   PaneCollection paneCollection,
   string layoutUri
)

//Find and close all layout panes associated with a specific layout.

LayoutProjectItem findLytItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals("Layout"));
Layout findLyt = await QueuedTask.Run(() => findLytItem.GetLayout());  //Perform on the worker thread

var panes = ProApp.Panes.FindLayoutPanes(findLyt);
foreach (Pane pane in panes)
{
  ProApp.Panes.CloseLayoutPanes(findLyt.URI);  //Close the pane
}