I'm using the NTreeList with build 11.12.14.12.  One of my columns is specified like this:

col = new NTreeListColumn();

col.Width = 25;

col.Name = COLUMN_COUNTRYIMAGE;

col.Header.Text = string.Empty;

col.Sortable = false;

tl.Columns.Add(col);

The column shows images on the nodes and the column header is blank.  Though I've specified Sortable to be false when the user clicks on that column header it will highlight the column and show the appropriate ascending/descending glyph.  How do I stop this sort activity from being allowed?